I think the best and only really good way to practice snapping is to just snap against someone in the bunkers you'll be playing. Shooting targets is good, it teaches you aim, but its not quite the same as getting shot back at. You have to take into consideration the times on the field that snapping is important.
Your in the snake. You walked the field earlier and know that when you pop up your gunning for D2. Aim is extremely important, once you take those first few shots, if they miss, he tucks in and you lose your opportunity.
Practice in practical positions that you'll play. Coming up to shot from a snake position is different than snapping out of a dorito.
Timing is equally as important as accuracy. You could have the best shot in the world but your not gonna hit your mark if you can't time it right. You gotta have a ball in the air just as he decides to come out. Then he can catch it with his face
In the absence of a field or other people though, anything works. Just dont pass up the opportunity to do some real life practice snapping.