These are a few things I'm preaching, though didn't necessarily practice...
Get more sleep and rest well – it makes all the difference
Do a practice race - a half marathon or something just to get used to race procedure
Know your route
Have a pace watch - you lose track of everything, and if you want to keep time then it's vital
Strategise - a plan of action is no bad thing
Plan how to get to the location properly - my fundamental error, and not one I want to repeat
Make sure you have someone to cheer you on at miles 18 and 23 – mentally vital
Drink - take a swig at every drink station
Flip flops - a kit bag essential
Be prepared to want to run another
