I used to count out my paces on the ground and mark when I should start counting! Then under saddle it was much easier to follow through! Remember to have a good balanced position and clear directions to the horse!
The trainer at my barn has a wonderful method! We call it "the three P's" Pace, Path, and Position. if your pace is consistent, you ride a good track, and your position is good, the distance will come to you!
You can also train your eye while you ride around the arena in canter and on the long side of the arena you take something you see on the ground or on the side of the arena and imagine there would be a jump. You count (start counting in the corner) how many strides you need to get there.
Then you can make it more difficult ( when you need usually 5 strides to the imaginary jump) you tell yourself to only need 4 jumps to the imaginary jump and ride forward, then need 6 instead of 5 and go slower.
I am from Germany and my trainer who is really well known here said "a good rider should ride at least 100 jumps a day to practice the ability to see the distance. But because jumping in reality would be bad for your horses legs if you do that everyday you should do it imaginary" with this exercise you train also the lightness of the horse.