Yeah this is pretty much just "known" by all BG'ers. You can jump on the flags to annoy melee.
So to add use to this post, here's a tip:
Shadowmeld! They will cap and move on, usually. Wait for the right time to right click on the flag and ninja it back. Very useful unless the hordie is smart enough to spot you or if someone with invisibility detection spots you. Cracks me up all the time.