Posting a follow up to Nicole Simon’s Ads: You can post them later! I thought I would mention a common practice used by membership sites or other types of sites that require registration, such as message boards. You can easily show ads just to those who are not logged in, leaving those who are regular members without any ads at all.
Many publishers running AdSense on phpBB message boards use this technique, since you can use variables or different templates to give one set of users (ie. Guests) AdSense or other affiliate ads. Then you can specify that registered or logged in users (or even a specific group of registered users) do not see any ads at all. Or while guests might see 3 ad units and an Ad Link unit, registered users only see a single ad unit or an Ad Link unit.
You can also set up custom channels and track what registered users and guests are bringing in to your AdSense account. You might find guests are responsible for 85-90% of your AdSense income from that site, so it would make a great “feel good” community gesture to remove AdSense from those who are registered. And you can use an ad free environment as an incentive to encourage users to register, if you are striving for increased numbers of registrations.