Conserving space on your Micro SD Card

Does anyone here have any tips or suggestions on how to conserve space for your card so that you can fit the total amount of games as much as possible?
Oh, and if I have the RAR and NDS files for anyone of my games do I still need to the RAR format for future use, or can I just delete it?

You can safely the delete the rar files.I don't think you can do much to the nds files except trimming them, the NDS doesn't have native support for uncompressing.Use SkinnyROM for trimming ROMs

Will trimming it have any effect on the performance of the rom?

Not as far as I know, all it should do it get rid of excess data which is sometimes present.

Oh okay.
Sankyuu