Post your Hold The Flag stratagies and tips here (This first post will be updated with time with the incoming information so stay "tuned")
I'd have to say, best idea is to try and stay on the top of the maps, especially the round ones (don't know any names off the top of my head). I try not to go to the bottom unless absolutely necessary, since it's hard to get away and you tend to get nade spammed.
My tip stay together when you hav the flag so if your bearer gets killed someone can quickly run in and grab it. Also in a grounp keep the guy whith the flag in the middle so it is hard to kill him.
Since most of the default HTF maps have "player-collide" only polygons, passing is very useful. In the event that you're about to be killed and your teammates aren't near you, you might still be able to throw the flag at them through a polygon.
This also means that it's not usually good idea to sit upon these "player-collide" only polygons, seeing as how you could be shot in the ass before you coulde react.