While we're on the topic of attire, don't wear that year's race shirt on race day.
Aside from the immediate marker that you're a newbie runner, there's actually a much more important reason: you shouldn't be wearing anything new. Yep, not even a T-shirt.
No matter what cool gear you scored in your registration bag or at the race expo, you have to resist the temptation. This goes for socks, running belts, body glide, compression sleeves—anything. Unless you've tested it during training, race day is not the time to try out new things.
Lastly, whatever you do, don't you dare put on a new pair of running shoes.