Ok I have used Barbed wire on my chaos marines to emphasise they fact they fight in cites a lot.
Things you will need.
Florist Wire (this is really cheap, you can do a whole army with 1 roll, it costs £1.56)
PVA glue
Whatever you use to base
Old paintbrush
Boltgun Metal
Mithril Silver
WARNING, THE WIRE IS SHARP

(ouch!)
1. Grab the Paintbrush and the wire, and reel off a small amount, then start to wrap the wire around the paintbrush.
2. Once happy with the amount, Grab your clippers and clip the wire off the reel leaving it on the paintbrush.
3 Pull the wire off the paintbrush and expand if neccesry, Once done clip off the amount you need for your base. Normally you can do 3 bases on one go.
4. Once based Put a small amount of PVA glue on the base and put the Wire down on it.
5. Grab a good paintbrush and paint it Boltgun metal.
6. Give the wire a light drybrush of Mithril silver.
And you are done, it should take 10 minutes top! You could use this between posts to make barbed wire terrain.