Something that hasn't been mentioned yet, is how your horse travels. If he is going with a stiff or hollowed back, it is much harder to sit on him as he is not carrying you.
A horse that is working over the back and is swinging, will be much easier to sit on.
You can also slow the trot, when I am struggling to sit a big moving horse, I will ask the tempo to slow to the point that I am able to sit the trot easily and absorb the movement through my pelvis. Once I feel comfortable at that tempo, I'll ask the trot to come a little bigger and so on, until I'm at the trot tempo that I want.
I find that once you lose your rhythm there is little point trying to continue, as you'll bounce further out of the saddle! So put in a quick rise and then go back to sitting and start again.
Sitting trot out of walk is also much easier, than trying to get the trot going and then sit it.
But as others have said, your core strength is the most important part of being able to sit trot, along with the ability to 'work' your pelvis. Some people (like myself!!) are not so loose and open in the pelvis, and as a result have to work very hard to develop the looseness necessary to sit a big trot. Some though, just naturally have an open hip and pelvis and seem to be able to sit any horse. The length of your leg does play a part in this, my legs are very short (bit sad I think I'm the shortest dressage rider in my state :P) and I struggle to open my hip and pelvis, as do a lot of my 'vertically challenged' friends compared to my longer legged friends who all seem to have a naturally loose pelvis!