It really doesn't matter that much what you practice with as long as you practice. Shooting basics remain the same. Deer generally don't stand still and give you a chance for a second shot if you miss the first.

Try to find a gun club that shoots a reduced distance high power match. The guys and girls will be more than happy to help you with the different positions. Reduced matches can be less frustrating than a full 600 yard match.

Spend your time practicing mostly in the same way you will hunt. If you will be creeping through the brush, then practice mostly shooting offhand. If you will be hunting from a ground blind, then practice mostly from a sitting position.

Although deer are relatively easy to kill, it matters mostly where you hit them and less what you hit them with as long as you shoot a proper bullet. Although a .223 will kill them, a larger caliber will give you more leeway if the bullet placement is off.