I hate to be confusing but I do not agree with the 'knees and thighs' posters. Thighs, yes, but never grip with your knees. This will tip you forwards and encourage the horse to go faster, and not only that but it'll make your legs very unstable. It's a bad habit to get into. Your knee should always be loose against the saddle; instead, grip with your thighs and calves.
HOWEVER, be sure not to grip too much at the sit trot. You cannot just hang on with your legs. It's uncomfortable for your horse and it restricts him. When I was learning to sit trot, this is what my instructor aways told me: Imagine your horse is a tube of toothepaste. You only want to squeeze a little bit so just the right amount comes out, but you don't want to squeeze too much or the toothpaste will come out in a giant glob. It sounds silly and childish, I know, but it does help.
At the sit trot, you can sit slightly farther back then normal. Keep your shoulders tall and your heels down. Loosen your hips and let them go with the horse. You should actually be able to feel which legs your horse is moving forward at all times.
I thought wetrain's excersice was very good. I've never heard of it before, but I can imagine that it would help a lot. You can try the monkey strap but it's always good to have control of the reins, especially if your horse tends to run aways with you. Half halt in the corners if he gets too speedy, sit back, and do lots of bending. I'd also suggest asking your trainer to have you trot on a lunge line.
Good luck. Hope I was helpful. :)
-----In riding horses, we borrow freedom-----