Pull your chest towards the bar and try to pull your shoulder blades together. That is going to be the foundation that you're going to press upon. Place them and your traps down on the bench...If you've done this correctly, you should have your eyes under the bar, you should be up on your traps/shoulders, you should have good arch in your spine... and then you place your butt down to set the position. If your butt sticks, you're not going to move. If your traps stick, you're not going to move. If any of that is wrong, you're going to be sliding around the second that you unrack that bar.