i use a big space to practice my trot out and i always have some one to help. you could try getting a nice, forard going walk and before your trot so that when you trot your horse is already thinking forwards and when you trot do not drag your horse instead tap them with a short showing cane on their shoulder to help to encourage them to extend and remember to run fast your self! Also try getting that trot in your ridden because that helped me lots because my horse already new that i want her to do that , if you get my drift. Thats all i can help with x