I'd bet that your legs slip forward because you're bracing against the stirrups for dear life! Next time you want to sit the trot, start out at a walk and bend your knees to pull your feet back underneath you; make your heels become level-- hold the stirrups with the ball of your foot, but don't use them very much or worry about jamming your heels down. Instead, think about your weight sinking down on to your seatbones. If you have to, take your knees and legs completely off of the saddle, note the feeling of really sitting in that saddle, and try again!
Before you ask for the trot, think about the motion-- it'll be a 1-2-1-2 motion that initially feels like it's shoving you forward on each beat, but instead think about the trot moving each seatbone independently- left, right, left right in time with your horse's hindquarters.
Now that you have the beat you want, gently gently gently! ask for a baby trot. Ask with your legs, but also by starting to gently move your hips in the left-right-left-right beat that you're asking for. When the horse eases into that trot, just relax! If it gets overwhelming, come back down the walk and relax.
The other keys are relaxation, practice, practice, relaxation, and also practice. ;) I've been working on the sitting trot for a year and am just now starting to become proficient! Good luck & don't get frustrated!
Last edited by existentialpony; 10-21-2013 at 11:05 PM.