Here's a tip I got from someone on another forum when our cornsnake had a bad shed due to the type of viv he's in - Exo Terra - the type with a lot of slits in the lid (solved that problem by putting Clingwrap over it and just leaving the front vents open).

If your BP doesn't shed properly let her soak in some tepid water for a little while (5 - 10 minutes). Get an old towel which is on the rough side and wet it. Wring the towel out so it's not dripping wet. Hold the towel and the snake and let the snake slither through it. Keep a firm grip on the snake and the towel (but not too tight) and hey presto! Remaining shed comes off