First of all, as I am sure you already know, make sure you're cans/jars/lids are clean and sterilized, fruit is peeled, etc...

Chances are you may have to add syrup to jar. A "syrup" could be just sugar and water boiled to form a simple syrup, or even a fruit juice sweetened with Splenda. The syrup actually acts as the preservative.

But here's something that helps a great deal. Add 1 tablespoon of bottled lemon juice per pint volume of jar or can. Add to the jar just before the fruit goes in. The lemon juice will increase the acidity of the mixture, this will prevent any bacterial growth and help preserve color. It seems a little odd but consider this, one of the reasons tomatoes can so well and keep so long is that tomatoes are naturally very acidic, so when they are canned their own acidity helps preserve them.

Good Luck!