You can improve your technique a lot by carefully watching Mark Rippetoe's Starting Strength DVD. Ask someone to record video of you lifting every now and then and get a feel for your body in space or upload it to youtube for advice if you're unsure.

Mostly by overstretching particular joints under load, eg. behind the neck presses and pulldowns & pushing myself on dumbbell presses with weights I could move but barely control, at one point I lost control but did not drop the weights.

The others where loading up machines with weights I could not have moved if it weren't for the fixed movement plane of the machine, that ultimately meant trouble for my lower back (leg press) and grinding in my knees (leg extensions).

btw, most personal trainers are horrible at teaching proper technique. I'd recommened the books written by Marc Rippetoe and Bill Starr and watching videos by powerlifting coaches and lifters (that do not focus on lifting in gear). I could 'fix' (read: strengthen and work around) my back troubles with deadlifting, but would not recommened that to someone without a guide or knowledgeable help at hand.