I learned a lot by recording tracks of my online fights and watching them immediately after getting shot down, while the action was still fresh in my mind. Watch from your point of view to figure out what went wrong, and also watch from your opponent's point of view to see what kind of target you presented to him.
Also, I really liked this guide: "In Pursuit, a Pilot's Guide to Online Air Combat" (
http://www.virtualpilots.fi/feature/.../inpursuit.pdf)