[quick99si]

Decent rom (one's own is best!), disabled push internet, QuickMenu instead of WM6.x start menu, tweaked cache/glyph caches, etc.. should get you to around 50% usage with Sense enabled, or something like 33-34% without it.

You can also turn off the proxy memory cache that will help your operating RAM, but this comes at a cost of a slight delay when starting any application on the device. I tried disabling it for a couple of days and couldn't take it so I went back.