For running line to mono backing, I use a nested nail knot. I like it because it's small and has a nice taper to both ends, helping it slide through the guides, even more so after finishing it with a little UV Knot Sense.
If you haven't tied one before, see Anil's instructions below from another thread. Start at STEP 2 and metnally replace "Dacron" with "running line."
"This is a very effective connection for fly line to backing, and is the one we use the most here at the shop. It takes a little more time but that connection (when it’s needed) can be extremely critical.
STEP 1: Take a section of thin backing (20lb. works best) and tie a two turn nail knot onto the last 8 inches of fly line. Pull this tight. It will seat and begin bighting into the PVC coating of the fly line. If you continue to pull, the knot will strip this plastic coating off of the fly line.
STEP 2: You now have roughly eight inches of Dacron core exposed on the back end of your fly line. Take this material and tie a 5 turn nail knot onto the backing of the reel (approximately 10 inches onto the backing).
STEP 3: Flip these lines 180 degrees and now tie a five turn nail knot with your backing onto the core of the fly line.
STEP 4: Pull on the standing ends of both the fly line and your backing. The two knots will slide towards one another and seat or ‘nest’ with each other.
STEP 5: To make this already small and unobtrusive knot slide through the guides even easier, coat it with U.V. Knot Sense (or similar glue) and you are done.
A few notes: This knot is extremely strong and slim, making it perfect for backing to fly line (since this knot will hopefully be flying out through your guides). You should use additional turns if you are using GSP backing as it is more slippery.
Anil"