You're asking folks to give up their most prized secrets... Oh well, I'll tell you if you promise not to tell everyone else.
I think everyone does it a little different, but here's some of the things I do. I change speeds. Sometimes you can actually slow down and your opponent will pass you. I don't fly in a flat circle. Try to imagine you are drawing an atom with your plane... Like this
I also try not to keep the turn as tight as possible. Most people hold their turns as tight as possible and fly in an almost perfect circle. I try to flatten out in parts of the turn so its more of an oval than a circle.
I will also fire random shots, just a two or three shot burst. The opponent may hear it and think you have the advantage and change their pattern or direction.
I'll also barrel roll a couple times to change my direction, like hamers said.
Finally, I do not ever recommend dropping your bomb, but if they have, they have the advantage. Try to get them while you're still carrying. It is possible. If you practice while you're carrying, it only makes you that much better if you've dropped. If you can't get them in a couple turns, you might as well dump your bomb, at least until you improve your chasing skills.
Hope it helps. Remember, it's a secret just between us. If I hear you've been blabbing, I'll be really upset!