NOTE: My router is a Linksys, so if you have a different brand, some of the settings will be in different places.
IMPORTANT: One time as I was doing this, I accidentally locked my own computer out of the router. If this happens, don't panic. All you need to do is connect your computer directly to the router using a cable and change the settings. The MAC security lock will only keep computers off of the wireless function, so you won't permanently lock yourself out.
Step 1: Wireless MAC Filter
Next, access your router's settings. The standard Linksys address is 192.168.1.1. If you don't have a Linksys router and don't know the address, you can probably find it online.
Go to the WIRELESS tab, and access the WIRELESS MAC FILTER panel. Select ENABLE and then PERMIT ONLY. Don't save settings yet. Click the EDIT MAC FILTER LIST button.
Step 2: MAC Address Filter List
Step 3: Adding Computers
Step 4: Save Settings
Double check that the radio buttons are set to ENABLE and PERMIT ONLY and then hit the other SAVE SETTINGS button.
Step 5: What the MAC Filter Does...
This security isn't perfect. MAC Addresses are burned into the wireless client's hardware, but some allow for soft-changes to impersonate other addresses. I've seen websites that claim MAC filters are easy to get around with the right equipment and know-how. I've never had a problem, but if security is really important to you, you can combine this with other security measures.
The benefit of NOT adding other security is that your wireless connection appears as unsecured, which is nice if you want to boot a leech in a clandestine manner. As long as the leech isn't familiar with MAC address hacking, you'll probably be fine.
For more info check out Wikipedia, it has a good article on MAC Addresses. Also check this out to see the weaknesses of MAC filters.