I've noticed that some "other" forums use contests or competitions for hardware or software prizes to encourage new particpants and get the posting levels up. This has actually been pretty effective at times (see how the current Gmail contest has stirred things up!
Many software publishers and hardware/accessory providers are willing to donate prizes for competitions when approached, so this might be an option. Competitions could be structured any way you liked.
The site has already evolved so far from it's beginnings (I've been around a while
) that I think most improvements have already been done. Mobileread.com is a pretty mature and well developed site (and I really like the lack of banner adds, YAY!!!)
So that's my $0.02 anyway...