1. Keep the ball in front of you. Don't defend outside of the 3 point line. Scouting report must be used to know who is a shooter and must be guarded outside the 3 point line, making a decision to give up 2's instead of 3's to a really good shooter.
2. A pack line is probably better than regular man to man rules with help side. Along with planting a man in the lane, a half man/half zone type look. This way when an undersized team has to rebound, they have their man.
3. An undersized team who is not good at D, must block out...and many times illegally. It's not a foul until is called, then called again, then again; well you get it. They must keep their player behind them and hold them if they have to by wrapping them up from the front.
4. Then they MUST get every defensive rebound especially those that go long or hit the floor. You have to compete like never before to get those rebounds.
5. The best defense is often your offense. If you are patient or can score at ease, it helps, but you must be prepared for those nights when you can't score, and then you had better not turn it over and you must find a way to get some offensive rebounds.
Mental toughness. You must do these things all of the time and get every rebound, or you will be in for long nights and/or seasons.