Have your bike well serviced before a race to make sure it is in 100% condition. Do this at least two weeks before the event such that you have the opportunity to ride it and check out after the service!

Pay special attention to the chain, cluster, derailleur, BB and wheel bearings. Preferably use sealed or semi-sealed bearings.

Make use of water-repellant grease for packing the bearings and headset etc.

If in doubt, change a component before the race or take a spare along as four days of riding really puts your bike to the test.

Spray an anti corrosive product into all holes in the frame tubes.

Seal all holes in frame, and around the seat post etc with silicone.

Make use of a good chain lubricant. Preferably use a dry lube like White Lightning or Finish Line.

Use tire liners or Slime.

Recommended spares:
To carry with you during the event

Spare tube
Puncture kit
Chain breaker
Allan key set
Chain lube
Roll of duct tape
Pump

Recommended bike spares to be carried on the back-up vehicles. (Check that it is the right one for your bike)

Spare chain or master link
Spare tire
Second tube or patch kit
Spare brake and gear cables
Spare chain lube
Bearing grease
Cables
Bearings
Axle Spanners
Cheap or secondhand set of pedals
General tools

Good Luck!