I've been thinking about physical security a lot, mostly because I happen to be revising chapter 5. Take a minute right now to look around and see whether your physical security procedures are adequate. Could someone easily walk off with a server? (If someone can steal a DC, they can 0wn you totally, basically forever). Do you have adequate environmental protections-- power conditioning? heating/cooling? fire warning & suppression? I could write on and on about this, but I bet that if you spend a few minutes thinking about your environment you'll see what you need to do to improve it, probably at very low cost. The US Army's Field Manual 3-19.30 has some interesting thoughts that may help you.