What you choose to do in the few hours before a race may not fully make for a great performance, but it can certainly break one. For example, having a few martini's is probably detrimental, as would be eating a 1/2 lb burger and a basket of greasy fries or some huge breakfast. Now is the time to realize your college buddy who called a pint of Guinness "the breakfast of champions" was being facetious. Prayer, meditation, warmups, and ritual bowel movements (whatever floats your boat, right?) are examples of things that probably won't hurt, but anything that impairs your mind and body or drains your energy reserves should be avoided.
I've gotten into a routine of waking up 3-3.5 hours before the start of races, and eating something carb rich and easy to digest immediately - only about 200-250 calories. Usually it's hot cereal made of brown rice farina, which is high in complex carbs (sustained energy). Then, I drink a couple cups of coffee, followed by 3-4 glasses of water to fully hydrated by start time. I make sure not to drink anything or about 30 minutes before start time, so I don't have to go during the race. I do this because well, I want to top off my energy and hydrate right before the race, but don't want to have a bunch of stuff in my stomach when I get to the starting line, because that can cause side stitches. So, my pre-race routine is based on some kind of technical rationale, because that's the way I am.
I know a lot of others don't have a routine, or perhaps they just think they don't, as pre-race practices are frequently the subject of discussion in online forums on running and cycling. Most people seem interested getting the nutrition right, because they feel like races drain them of energy.
But seriously... It's a race. If you're not getting fatigued and pushing through it, you're not racing. I think the reality is that there is only so much you can do in the short few hours right before a race that will really vastly improve your performance, but there are probably things you could do that would really hurt it.
For me, it's all about conserving what you've got, and maybe trying to top off your energy and hydration in a rational way before the race. I don't really go overboard with much else.