obDisclaimer: Executing the following is almost definitely illegal, no matter where you live.
- Case the target
Pick a street sign. Signs in residential areas work best, since they are in low-traffic areas. You'd have to be totally insane or very brazen to attempt to steal the "Speed Limit 65" from your local interstate highway. Metal signs attached to the permanent metal "U" posts are easier to remove than signs attached to wooden posts or to metal pipe posts or to temporary construction sign stands (unless the sign is just set in the stand, in which case removal is trivial), and wooden signs are a real pain to remove.
Pass by the target a few times. Observe where it is, what cover is near it, lines of sight, traffic patterns at various times of the day. Make sure you know what kind of bolts are holding it on, how many there are, and where on the sign they are. The backs are always standard hex nuts, but the bolts differ. Some are smooth rounded, some are hex bolts, and some and rounded with a flathead slot. If this sign has one of the latter two, bring the appropriate tool for the front side; it will make the sign easier to remove.
Most importantly, notice how big it is and how far off the ground it is. Street signs are big; take the size you think it is and then multiply each dimension by 1.5 to get a more realistic estimate. They are also high off the ground (average 6 to 8 feet); this is why it's best to have a tall person do the removal.
Pick a time. Do it when the traffic flow is at its lowest, which almost always means very early in the morning on a weekend (4am on a Sunday works well in most cases). Plan everything you will do with the sign in advance. Estimate that it will take about 30 seconds to remove each bolt. If you are doing this without a car, what will you do with it when you get it down? Will you stash it nearby and come back for it later, or will you put it in a bag and attempt to walk away with it (remeber these things are big)? If you're doing it with a friend and a car, make sure s/he knows the timing as well.
- The hit (part one)
Approach the sign and quickly douse the hex nuts with WD-40. Leave. Rumor has it that the nuts and bolts that hold street signs onto the posts are designed to rust the instant they are attached, so even if the sign is new, the nut is probably attached to the bolt and post by a good layer of iron oxide.
Wait at least five minutes so that the WD-40 can do its magic. During this time, set up the primary monkey wrench so that the opening is about 1/2" wide. If you'll be using a second monkey wrench or screwdriver, get that ready as well.
- The hit (part two)
Return to the sign, which should now be ready for removal. Put the monkey wrench(es) in place -- always remove the bolts from bottom to top. If the sign has smooth round bolt heads, hold onto the head with your unused hand. Standing behind the sign, turn the wrench counter-clockwise to loosen the nut. If they don't start to loosen immediately, don't fight them. Re-apply WD-40 and wait another five minutes. The nuts should come off in about 8-10 turns. If there are more than 2 bolts and it looks like you may not have time to get all of the nuts off, take one off, and leave. Come back for the rest later.
Once you've gotten the nuts off the bolts, resist the temptation to remove the bolts from the holes as well. You need something to keep the sign from falling. Once you've gotten all the nuts off (don't just drop them; put them in your pocket or something), pull the entire sign off the front of the post. The bolts will come with it. Be very careful. Just as the signs are bigger than they look, they are also much heavier than they look. You will need two hands, and gloves may also help to keep the narrow, heavy edge from biting into your hands too much.
- The getaway
If you are lucky enough to have a waiting car, get the sign into the trunk and get the hell out of Dodge. Don't forget to remove the sign from your trunk soon. I had a friend who got pulled over, and when the cops searched his car, not only did they find the half kilo of pot he had stashed in the back, but they also found the 30-odd signs he had sitting in his trunk. Even without the pot, this was still a stupid mistake.
If you're stashing the sign for retrival later, get it out of your possession ASAP. Hide it well (a difficult thing to do with such a big, flashy object) in your previously-chosen location. Come back for it in a few days, preferably after the DoT has already replaced the sign. Finding a suitable carrying case for the sign is left as an exercise for the reader. Plastic trash bags are far too flimsy; the large canvas sack I once used on a paper route has worked fairly well in the past.
So now you have your trophy. Clean it off and present it.