Mend nicks and cuts in the top rubber of your road tires with crazy glue. If the cuts aren't into the casing fabric the glue will close them up and let you get some extra miles out of your tires.
Put a $20 bill in your seat bag or under the insole of your shoes for emergencies.
When changing tires, Start putting the tire back on opposite the valve stem to reduce the chance of pinching a tube.
Get out the saddle for 15-30 seconds every 10 minutes, even on level terrain to use the muscle differently and give you but a break.
Start to drinking as soon as you start your ride to get the habit of hydration set.
Quaker Oatmeal squares or Oatmeal on the go bars make an effective, tasty and cheap energy bar. Plus they are easy to open and chew on the bike.
In a pinch, Vaseline will work as chamois cream. When you are hot and sweaty it will reduce chafing and keep bacteria from growing in your pants. If chaffing does occur, you can use Bag Balm or Udder Cream to reduce pain and speed healing.
If your cycling shoes get wet, stuff them with newspaper to dry overnight so you can ride tomorrow without gunky feet.
Bring a compass when you are riding new trails that you don't know. Getting lost after dark sucks.
For an emergency kickstand, use the Velcro from your gloves to lock on one of you brakes. Now you can lean your bike against anything without it falling over.
For rides longer than 2 hours you need to eat and drink to keep going. A good rule is drink a full bottle per hour and eat 1.5 calories per pound you weigh per hour (240 calories for 160 lb rider).
Don't forget the sunscreen and bug spray.
If you notice anything loose, making noise or not working properly bring it in for a tune-up to be safe.