1. Strive to strike the rack at an angle.
2. If you have little angle, try to hit the rack at the corners, where it is "weakest". Don't get greedy and try to make something that isn't there. Hit the rack at a calculated spot to break 1 or 2 balls out.
3. In general be careful of using stun. I only use stun for shots where I really power the ball into the meat of the stack. Because the rack weighs more, the cueball will bounce back on these shots instead of sticking. If you DO hit these shots to weakly, you will stick! You can stick using stun, even if you hit the rack at an angle, if you happen to catch a ball full. This principle is used to play full rack safes!
4. I think it is helpful to have a little spin on the ball in the direction you are going. If you are going forward, top spin, if you are going backward; draw. That way, if you do happen to catch a ball full, the cueball will motor on and break free most of the time. Also the bending path will sometimes give you that extra bit of angle you need. Even a near hopeless breakshot can be made viable if you have a good stroke and use lots of spin. Watch snooker players break the stack with the black...
5. Remember to factor in the "bend" caused by top or bottom spin. Sometimes the ball will bend in way that causes you to hit the rack perpendicular, instead of at an angle. This is IMO especially true when you have the breakball close to the rail at the side of the stack, and the cueball out in the middle. These breakshots are very dangerous, if you don't know how to play them.