Weasel boxes is all I use to catch weasels. They work great and are easy to make. If you look in the archives there is info on how to catch weasels, how to make boxes and so forth.
Also Weasels love blood! So the bloodier the bait you use the better. Some baits that I use is chicken liver (sometimes it helps to mix some anise into it), mice, squirrels, muskrat meat,etc...
Some people put straw and grain into the boxes to attract mice into them which then attracts weasels to the box. ( I wouldn't advise this method if you are using rat traps)
For traps you can use 1 longs, 1.5's, 0 longs, snap rat traps and any other smaller sized traps. I use long springs when not using rat traps because they tend to go off a lot easier than a coil spring trap. Especially since the weasel is so light footed.
To help catch the weasel that is getting to your chickens you can build some boxes and put a few in the area of your chickens. It shouldn't take long to catch one, especially if it visits qutie often.
Here are some links from the archives on weasel trapping:
Weasel boxes Weasel Boxes
The Weasel Thread The weasel Thread
Weasels-2008 Thread Weasels-2008 thread
Milk jug weasel set Weasel in a milk jug set