Making more space
You might have thought that it was only possible to hold 48 pokemon on colosseum. However i have found a way to fill up all your boxes so you can hold 90 pokemon. This takes some time but it is worth it.
1. First catch a load of Nincadas on ruby or sapphire.
2. Trade them to colosseum.
3. Evolve them, make sure you get 1 Shedninja and 1 Ninjask.
4. Vwala you have two pokemon from 1.
5. Just keep doing this until you fill all the boxes.
6. Then trade what ever pokemon you want.