I'm still relatively new 'round here (end of November) but I've been playing sims for a while now, so I hope this helps:
A break turn (a flat, hard as you can yank the stick back, horizontal turn to the left or right, or as I think you're describing it a "U" turn) is really a maneuver of last resort, and it's not a reliable way to get out of trouble unless you're in something like a Spitfire or Zero, which can 180 on a dime with nine cents change. It'll bleed airspeed, and a good opponent, or certainly one in a better-turning plane, will probably have little trouble following. If it IS your only option, try and pick up the position of whoever's following you and turn INTO him (either from his tracers zipping past your cockpit if they're on, or using your views). You'll fly through his guns, but he'll only have a brief snapshot before you're through and he has to maneuver to keep you lined up. Don't turn away from him or he'll be able to slide right onto your six with little effort
I'm sure other people will chime in with other suggestions, but I try to go more vertical, either a climbing spiral/turn or Immelman, (half-loop) especially if I've got some distance and airspeed to work with; or a diving turn, straight dive-out or Split-S (roll until you're inverted and pull into a loop) if I have the altitude. Some planes can tighten up their turns by using vertical turns called yo-yos (think of it sort of like a loop tilted at an angle. A high-yo goes up, a low-yo goes down). You can use this defensively to try and shake a guy off, or offensively if your opponent tries to break and you can't follow his turn otherwise (surprised the HECK out of a Spit XVI--and myself, for that matter--in an F4U with that a couple nights ago
).
If you're in a plane that rolls real well you can scissors until help arrives. There are a lot of variations of this--one is a series of break turns, first one way, then go the other way (the FW-190s and F4Us are among the best at rolling maneuvers like these, although these aren't beginner planes, certainly not the Corsair). It can help in several ways: The first is to keep your opponent from getting a good shot and buy yourself time. The second is that you can force an overshoot since you'll be shedding airspeed pretty fast. Third, if you're in a plane that turns better at low speeds (Spits, Zekes) you can bleed him dry and force him to either disengage before his controls turn to mush, allowing you to escape or turn the tables, or get him to try to press the attack and either stall out or put himself in a position where he's REAL vulnerable.
The key as mentioned before is practice! Read up on ACM, work a little on gunnery offline against the drones (not the best targets, but every little bit helps). Work with the trainers, and expect to be a bit black and blue for a while. It took me a good couple weeks to score my first clean (non proxy/collision) kill, and now I'm ALMOST useful for more than a target drone.
