Go to a stroke workshop.
I like to tune up my stroke at one of these. A swim team in your area should have one on a weekend morning usually once a month.
They will videotape you with an underwater video camera. Then you watch the tape of yourself and the coach will give you advice. Then you jump in the pool and try it out.
As far as enduance aids go......intensity during your swim workout will give you endurance. I highly reccomend joining some kind of masters swim team (masters is over 18 years old) That way you get a structured swim workout. Push yourself to make those intervals. This combined with some stroke workshops will immensly improve your stroke.
Also using handpaddles when doing pull sets (where you stick the foam between your legs to keep them afloat while NOT kicking) helps. You put incredible resistance on your upper body and it's a workout. I recomend getting the speedo handpaddles, they are shaped in such a way that you pretty much HAVE to use good stroke form or they don't really work.
Having a good kicking form is also important. remember you will get more power from kicking from your HIPS and keeping your knees basically straight and NOT moving. Let your ankles stay limp, the water will make sure they are in the right position to get the most power. THis is going to make your ass and hips BURN, but it really works.
Also try some ocean swimming if you can. Nothing makes you a stronger swimmer faster than fighting currents while swimming. Kinda think of it as freeweights (ocean), and you have to use stabilizer muscles over machine weights (pool) where it's nice and calm.
I hope i answered your questions.