I use 20lb UG or if I have some around, 20 lb fly line backing. I like the fly line backing in a bright, contrasting color so I can see it clearly if I'm getting close...

Make sure you use the right kind of knot...

Using mono or other type of backing will help save the expensive braid, help the braid not slip on the spool, and help fill out the spool better - to name a few.

So far I've been using Power Pro and Fire Line in the 10 to 20 lb range and really like it.