I usually make my pits level if the driveway is level or match the pitch of the driveway. Then start the missile slowly, keeping a level on the missile until it's all the way in the hole. The best thing to stop it from coming up is, DO NOT keep standing on the missile as it's going in the hole. Once the missile grabs the dirt get off of it.
And for a bonus tip on how to shoot a missile with laser guided precision over 50 feet, you're going to need two things. A shovel with a straight handle and a tamp or another shovel. But what you want to do is put the tamp or other shovel in the middle of the catch pit with the handle straight up. Then go over to missile with the straight handle shovel holding it like a plum bob and with one eye closed looking at one side of the missile and one side of both shovels all at the same time. Adjust the position of the missile until all 3 sides line up, preferably the same sides. Then start the missile and make adjustments if necessary.
