There are numerous ways to preserve leaves. No matter which method you use, it is important to remove as much moisture from the leaves as quickly as possible.
In the Microwave
Start by laying the leaves between two paper towels. If possible, set your microwave's power setting to medium. Now run the microwave between 30 and 120 seconds, depending on the dryness of the leaves (less time for drier leaves, more for those with more moisture). This will remove the moisture from the leaves, allowing the colors to remain as they are.
In a Book
First, find a big book, and one that you are not too attached to (this method may discolor the pages). Place the leaves (don't overlap them) between paper towels or blotting paper (this will absorb the moisture from the leaves). Allow at least 20 pages between the leaves for best results. Keep the leaves in the book for about 7 days, replacing the paper towels on day 2. This should nicely dry the leaves and also keep them flat.
Okay, they're dry, now what?
Now that you have dry leaves, their are a few things you can do. You can either leave them as is or add a bit more protection. This can be accomplished by placing the leaf between two pieces of wax paper. Place a dish towel over the wax paper and press with an iron (on medium heat). Press on each side for approximately 3-4 minutes. Remove leaf from wax paper. You will notice a fine coating of wax which will preserve your leaves.
You can also purchase a spray preservative at your local craft store. This will leave a glossy coating on the leaf. Important: This must be done in a very well ventilated area as the fumes are toxic. Children should not do this, please ask your parent or another adult.