Any kind of 'row' type exercise will work these. There's a good video clip here: http://www.realjock.com/article/656/
The key with these, I've found, is making sure that with whatever exercise you're doing to target the traps, you avoid calling on other muscles (arms/lats) as much as possible.
The best way to do that, again, that I've found, is to really SQUEEZE your shoulder blades together at the end of the movement. This might mean doing a lower weight than you know you can handle so that you are sure you're hitting them. You want to hold that position so that you feel the tingle of the muscles having to work before releasing and returning to the start position.
That's my un-certified/non-professional advice.