First things first is remove any applications and languages that you don't need. Whenever I load new firmware I always make sure to delete all languages except English. I don't keep a lot of apps on my 9900, and I have a little over 200MB free.
I delete unnecessary languages & any apps that I don't use. Pretty much don't mess around with the rest of it but I'm sure there are some bb experts that could guide you in that area. Good luck.