To dig 'em out, here's what I do:

First, get a probe. Grab a chunk of 3/16" gas welding rod, even 5/32" or something similar. All you need is a slender metal rod that is smooth. This is what you're going to use for a probe to find where the burrow really is. Take it to your bench grinder and grind a little point on it. Then weld a washer or a t-bar on the top. You're not going to use this thing to push real hard, but when you're pushing all day long if the gophers are getting away from you, it helps to have something larger than the other end of the rod pushing into your palm.

Next, get a slender trowel. I'm ordering a new tool from Gempler's, item W48, which is a combo knife/trowel, with a belt sheath/holster -- this looks pretty ideal. Absent that, get a thin garden trowel with a long-ish handle. I've also made a neat digger from a 2" diameter piece of steel pipe in the past, but it gets a little cold on the hands in chilly weather. A wood/rubber handle on the tool is nice. A tool you could carry on your belt all the time is better.

So you see a gopher mound on the field. Here's a high-res photo of a pocket gopher mound, what I like to call the "classic presentation":

http://ag.arizona.edu/yavapai/photos/gophermound.jpg

OK, do you see that circular area of dirt in the lower part of the mound? That's where he surfaced from the ground. Now, moving downward and slightly to the left of the center from that round area is where the tunnel is -- the dirt "fan" of the mound is typically opposite where his tunnel is. Think about the mechanics of this -- his body has to be somewhere in the tunnel as he's pushing all that dirt up. His tail will be down the burrow, on the side of the hole opposite to where the fan of dirt is. The tunnel is probably about where that small stone is in the picture at the 7-o-clock position on the circular area of dirt.

So take your probe and start sticking it into the center of the round center of the dirt mound - you should be able to define the edges of the hole by working outward in all directions from the center. Now start working your way with the probe (lifting the probe and sinking it in again) at 0.5" intervals, working your way back along where you suspect the tunnel to be. You'll learn how a tunnel "feels" on your probe -- you press in, meeting resistance, then about 2 to 6" down, the probe is very easy to push in for 1 to 3", then it gets harder again. The "easy" part of the push down is obviously where the tunnel is.

Once you get experienced at this, you'll learn how to "read" a mound and you'll start being able to stab your probe down into the tunnel 90% of the time on the first probe. You want to find which way the tunnel is heading away from the mound, because that's where you'll have the most success digging in to get a clear, open tunnel to where the little SOB is hiding. The gophers typically stuff the last 3 to 6" of the tunnel closest to the mound, so if you want to get a trap or poison into the tunnel, you have to pull all that dirt out. If you probe to find the tunnel about 8" away from the mound, you'll be in open tunnel and this is where you want to put a trap or the phos-toxin with the least digging.

Now, in a field that has lots of gopher mounds, you'll be wondering "which mound should I dig?" Ah, this is where you graduate from being a novice to a master gopher killer. Gophers push up lots of mounds, and without anything to knock them over, you'll just see a bunch of mounds in the field. In the spring, I highly recommend going over the field once with a blanket harrow to knock down all the mounds, then go out in the field the next day to see all the fresh mounds -- trap/poison those new ones. But let's deal with the "during the season" issue where you've just spotted a dozen new mounds since the last cutting -- typically only TWO of the mounds you see will get you close to the gopher. Gophers are always expanding their tunnel system, but they rarely move backwards into the "old town" part of their tunnels except to kick out any interlopers and foreign gophers. Gophers are anti-social, except in mating season, so they will not tolerate another gopher in their tunnels. But they're constantly expanding their tunnel system in new directions, looking for new roots to eat -- and this is where you'll find them. If you look at a system of mounds early in the morning in the summer, you'll be able to spot the new mounds easily -- they'll be the only mounds with fresh, damp earth on top. When you get more experienced, you'll be able to read a collection of gopher mounds and see which ones are most recent by looking at the shape of the mounds and the location of the mounds in relation to each other. The new mounds will be the ones with other mounds on one side, but clear, clean field on the other side -- this is the "working edge" of the tunnel system. Find the two freshest looking mounds, probe and poison/trap these mounds. You could put traps down the other 'n' mounds in the system and never get a hit unless you somehow draw the gopher back over to those areas of the tunnel with bait -- like a carrot or something. 

So you've got the active mounds located. You've probed the mound(s) to find where the tunnel is. Open up the tunnel, going straight down for poison, or digging a larger area for a Cinch trap. Be careful when using your hands to clear out dirt -- I'd recommend wearing a leather glove on the hand you're using to sweep dirt up out of your hole. Gophers can (and do) occasionally bite, and believe me, a bite without a leather glove smarts and draws lots of blood. The one that bit me did not live to talk about it. I tore apart the ground with a pick and shovel to be able to kill the little SOB with my boots. He was a wet spot on the ground when I got done. Still, it took a couple weeks for the wound to heal up.

OK, you have an open hole into the tunnel, and you can feel that the tunnel is truly open. Drop in 3 to 6 of the small tablets, sorta tossing them back into the tunnel away from the mound, only one to two of the large tablets, then spray (from a backpack or ATV sprayer), oh, about as much water as you could spit in one mouthful onto the tablets -- a few CC's. That's all it takes. Some guys will tell you to use chlorine bleach to "speed it up" -- that's not necessary at all. The moisture in the air will activate the phos-toxin, and the dampness of irrigated ground definately sets them off  -- the liquid water is just speeding things up very fast and will cause a rapid out-gassing of the tablets in about five minutes. When the tablets are done, all you see is a formless little pile of mush where the tablets were.

The phosgene gas is heavier than air (much) and will sink into the tunnel system. I don't even bother covering the hole unless I'm dropping the tablets into the holes early in the day and it is windy. I usually go dropping phos-toxin late in the day, when the winds are dying down and the critters are down in the holes for the night -- this is especially true of squirrels.

Be sure to handle phos-toxin tablets with dry leather gloves -- NOT your bare hands. There is no antidote for phosgene poisoning. Do not store these tablets, even in their aluminum flask, where you or pets will be exposed to any gas from a loose flask plug. If you smell a "garlic" odor where you're storing them, it means you have a leak or some spilled material.

What I like about phos-toxin is that it works, works well, without secondary kills, and I can use the same material on gophers, squirrels, voles, badgers, coyotes, foxes, etc. If it is in a hole in the ground, I can kill it with phos-toxin and not worry about non-target species. So I carry a flask and a trowel in the work pickup wherever I go, all year long. It is my all-purpose killin' potion.

The best insight I can give towards elimination of gophers and squirrels is to kill them whenever you see them with whatever you have at hand -- even your feet. I've stomped on gophers with nothing but my boots in the moonlight when I've been baling. I've swerved the balewagon over the top of squirrels and voles. I've used a .338 to kill a squirrel at 25 yards because it was all I had in the truck with me at the time. My neighbors think I'm nuts because I prefer shooting squirrels with a 1 ¹/₈ oz load of #6 shot out of a 12ga. Wanna know something? It works. No squirrel has ever walked away from a load of #6 shot launched by my shotgun. Wanna know something else? When all the young squirrels are huddling around their hole in early June, one 12ga shell delivered at the right range to get about a 30" spread of the shot can kill six+ with one shot. Been there, done that, several times every year. I've killed gophers early in the morning with a .22 -- you can see them pushing dirt up into the mound just after dawn and if you have sharp eyes, you can see them mounding the dirt. Aim for where the hole is in the mound, wait until you see the dirt being pushed up and pop a couple rounds into the mound at a high angle trajectory, like you're standing up on the seat of an ATV parked near the mound. Dig him up and you'll find you've blown his face through his throat.

Want to know what I want most in ag equipment cab design? A place to put a good carbine or short-barreled shotgun safely secured. All these ag equipment designers must farm in a three-piece suit, never getting their hands dirty. I need a place to carry traps, guns, trowels and "stuff." I have no idea why equipment vendors brag about their cabs. There isn't one of these outfits making a decent cab on any piece of equipment for a working hay farmer. They're putting in seats for the banker, but you can't get a door or window open fast enough to get a shootin' iron out there to kill something, because you don't have a secure place in the cab to mount a long gun. Trying to get a shootin' iron deployed looks like a three-legged man in an ass-kicking contest in an older JD cab. 

My mentality is that I don't make an excuse to "deal with it later, when I have the right tool/trap/gun/bait/poison" -- I kill them right now, when I spot them, because I want them dead now. Their reproductive cycle is too short for delay and procrastination. When you kill one female gopher in April, you've prevented something like 40+ gophers by the end of the season.