Before a big race don't...
1. over hydrate (drink plenty of fluids, but not to excess).
2. fill your last taper week with cross-training or strength training or trying to get in those missed runs (your body needs this time to repair so you'll be refreshed for the race).
3. eat pasta with a heavy cream sauce the night before the race (waking up with indigestion is never a good thing). Actually, you don't even need to eat pasta. Eat what you've been eating the night before your long runs. Yes, it's great for your pre-race dinner to be simple-carb rich, but that can take many forms. You know your own stomach. Don't try something you've never eaten the night before a long run.
4. go to bed too early (try to get 8 hrs, but going to bed too early may cause you wake up too early; then you may have a hard time falling back asleep). But also, don't fret if you can't sleep. If you normally get a good amount of sleep nightly on a regular basis, not sleeping the night before the race will not effect you negatively.
5. read back copies of your running magazines and discover all the things you should have been doing (avoid things that may cause you to question your training). Trust. Believe. Conquer!
6. listen to everyone's advice about how to run your race (you've put in the training; you know what needs to be done).
7. make last minute changes to race strategy plans (trust your training). Many races are lost in the first mile when runners get caught up in the race-start hoopla and bolt out too fast. Most PRs are made when races are run as a negative split meaning a runner starts out conservative and ends up running the second half a little fast than the first.
8. forget to layout your racing attire before going to bed, so you won't be panicked in the morning trying to find your clothes and gear (being prepared before going to bed will help your mind relax so you can sleep better). Lay out everything...shoes, shorts/tights, shirt/singlet, socks, race bib, hydration belt or handheld bottle (if carrying or wearing one), Gu, etc. If you plan to pack a drop-off bag to have at the end of the race, go ahead and pack that too. Let race morning be focused on fueling and dressing, not trying to find everything.
9. pick up a different brand of gel at the expo to use during the race, just because it was a great bargain or free (all gels are not created equal; never try something on race day that you've not tested in your training).
10. buy new shoes and/or apparel at the expo and decide to wear them in the race (blisters and chafing—that's what you risk by wearing new shoes and/or apparel; not a good thing on race day).