Holes. Lots of holes. I used a soldering gun to make them as they are cleaner holes and the plastic won't crack.
That is the key. I usually put about 5-8 on each side of mine, then wait a few days to see how things are going and add more if needed. You can get a soldering iron at alot of the dollar stores.
1.1 crazy dogs
4.3 even crazier cats
2.2 bps
2.0 Off Track Thoroughbreds
0.3 human kids
1.0 Boyfriend who puts up with the craziness