#1.
VST to RTAS adapter from FXPansion. a must have.
#2. your ears.
So many people think that "plugins are the answer". They aren't. Knowing how to make good sounds with what you have is really the key. While true, some plugins can make good tones sound fantastic, good sounds are in the hands and the ears of the "engineer" using them. There are two old sayings that come into play here:
you can't polish a turd
and
G.I.G.O (garbage in, garbage out)
A wealth of knowledge about how to get good tones (with and without plugins) can be had at
The Project Studio Handbook
(though I must say that
Analog Channel by McDSP is on the top of my "next to purchase" list.)