I recently have done battle with rats in my garage. Since usenet was a big help to me, I thought I'd summarize what worked for me and what didn't. I hope this info helps someone in the future.
The biggest problem (to me) is that rats carry many diseases, most unpleasant, some fatal. This fact alone may lead you to retain a professional exterminator. Unfortunately, it is my belief that exterminators don't *kill* many rats, but just drive them away. My infestation closely followed an episode where my neighbor had a rat problem and hired an exterminator. I think they just moved up the street. So, if you go this route, you may not be doing any favors to your neighbors by hiring an exterminator. If you DIY, then, wear a respirator and gloves, and wash your clothes thoroughly and often. I used a mix of bleach, ammonia, and soap and water daily.
IMO, traps by themselves are basically useless. Rats are cautious, and are pretty smart. At the least, if you use traps, bait them but don't arm them, so the rats become comfortable. Then arm them and you may nail a few if they are optimists.
The traditional approach is poison bait. The most common poison is warfarin, which is an anticoagulant. Unfortunately, rats evolve very quickly, at least metabolically, and have become resistant to warfarin. I had absolutely no food in my garage, but I found that the rats had made their nest in an old box of DCON warfarin-based rat poision (?!). A more modern poison uses bromethalin, but I am afraid rats are adapting to this, also. Bromethalin attacks the myelin sheath on the nerves (like polio). I never got bromethalin to actually kill a rat, but it slows them down so that other approaches become more effective.
Rats will take the bait back to their nests and hoard it, so I would recommend using cubes of bait (rather than loose grains) and then leave some cubes loose (for rats to take home) and fasten some down, so they are forced to eat those in place. After a few days, I began noticing rat tails in my driveway. The local predators were having a field day with my wacked-out rats. Also, my traps began to work as the rats were becoming disabled.
It seemed like the really big rats could tolerate the bromethalin the best, and never really slowed down that much, but at least by that point I had interrupted the life cycle by nailing the pups. At that point I played my ace, which is antifreeze. Rats apparently like the smell and the taste. The largest rat literally keeled over while drinking antifreeze.
I have been rat-free for a week, but have no delusions that the war is over. At least I won the opening round. I will be looking for additional poisons, beyond the bromethalin and antifreeze, and I plan to experiment with electrocution.
In summary, I would say:
*lead off with a bromethalin-based poison *keep rebaiting, and after a few days introduce traps *at first, use the traps unarmed, to breed familiarity, then *arm the traps. Continue to bait. At this point, warfarin may be of some use on already-weakened rats. *after a week or so of poisoning, introduce antifreeze to hit the biggest ones.
Then, clean up thoroughly. No food available at all. Close up any possible access points. Leave out bait blocks as a tell-tale.
What a drag. :(