How can I prevent wrinkles?

There are many factors that determine when and how many wrinkles you're going to get.
The first and most important is genetic. Look at your parents and your grandparents. Frequently the patterns of their wrinkles will be reflected on your face as well. You can use that information to determine where to concentrate your preventative methods. If everyone has deep 11 lines between the eyebrows, for example, you may want to think about Botox earlier rather than later to prevent the onset of those lines.
The second component is more under your control. Some of the things you do to your skin are not doing you any favors in terms of wrinkles.
Sun exposure - wear sunblock and don't go to tanning booths. C'mon - you know that.
Smoking - stopping will improve your skin and allow it to heal.
Dryness - use moisturizer and a good cleanser daily.
Oxidative stress - this one is a bit controversial, but it seems that antioxidant topicals like Prevage and others do help.
Making facial expressions - OK, I'm not saying not to do that, but over the years it will generate lines in certain areas (vertical lines
between the eyebrows, forehead horizontal lines and crow's feet are the classic ones). I'm also not saying to start Botox at puberty, but for
some people, preventative Botox in their late 20s to 30s (SMALL, non paralyzing doses) really keeps their face young.
Regular facials and microdermabrasion do help maintain your skin.
Don't listen to all the marketing hype about the newest cream or potion. Even if it has a doctor's name on it. Especially if some celebrity is hawking it on the Home Shopping Channel. Its a waste of money. Talk to friends you trust, doctors you trust and make up your own mind about a daily regimen you can stick to. If you keep it simple, you're more likely to follow a regimen.
If you just take care of your skin with simple things like these, you'll be far ahead of most people.