a few links for you
http://thegolfdrillguru.netfirms.com/nfblog/?p=301
One I use, and I dont have a link for it so I will describe it the best I can.
I take a 2 x 4 about 2 to 3 feet long and point it down the range. I put a ball somewhere in the middle of the board, about 2 inches away from the board, so that the toe of my club just misses the board.You cant come over the top ( and hit it outside in) with the board there