Server configuration for a shared server

I recently got a new server to provide free hosting to my forum members but due to the shared environment I have no clue what to do and what not to.

This is the server :
2X120GB SSD soft RAID 1

The problem is I have no clue how to make this server serve pages faster as I cant use something like an xcache or apc or even memcached and I want my members pages loading atleast better as compared to run of the mill shared hosting at the very least.

I installed varnish which I dont use for my own websites as I just dont like the idea of cacheing the page output but then what else can I do to make things faster?

I hope someone who has experience with shared hosting environment can point me in the right direction.