Is your horse in an actual extended trot, or is he just running out and on the forehand? You want to make sure he is uphill, with a soft swinging back and through. Once you have him like this at a normal working trot and can sit that easily, THEN start trying to sit the lengthen. I'd be starting by just asking him to step out for a few strides at a time then come back to working trot again. Gradually increase the strides of lengthened trot until you are comfortable sitting them.
If you find sitting a working trot easy, then you will just get accustomed to the larger lengthened trot and it will come to you soon enough ;)
Also, ABS OF STEEL!! The main point of your body holding you in sitting trot is your abdominal muscles and back, so you want to really focus on getting those muscles stronger, along with your core muscles.
I like to run quite a bit, along with a hell of a lot of sit ups/crunches etc. When you talk, hold your stomach muscles as well. I spend the whole day with my abdominal muscles tensed, so when I breathe, talk, walk etc. they are tightening all the time.
When you sit trot, try to think of riding you belly button up towards your hands, and another good saying I've heard is "push a baby out" :P Even if you havent pushed a real one out before, if you're female you'll get the general idea of it. It will help relax your glutes and work your stomach ;)