Silver Floral Wire
I've found that the best thing to use is floral wire, you can find it in most craft stores. Get a long piece of wire, fold it in half, and with help from a friend or a vise, twist the wire on itself. Then just get a marker (a somewhat thick one) and wrap the twisted wire around that. Then just mount it on some balsa wood and a base and viola, you have barbed wire. Try checking the Great Britian Games Workshop site for a guide to barbed wire, there may be another guide, I think on fortifications that this is based on. Also check previous black gobbo issues, I seem to remember something there.
Also it was in a WD, I wanna say it was 310, it was talking about the dark crusade.
Good Hunting
EDIT: I found a link, try this:
http://uk.games-workshop.com/warhammer40000/defence/1/
Nice article on fortifications.