1. Learn about Ron Paul's ideas. Read what he has written: there are over 900 articles and speeches by him at the Ron Paul Library. Read his Wikipedia biography. Watch the dozens of videos and documentaries that exist out there. In order to articulately support him, it's best to know exactly what his positions are, why he believes in them, what you agree with him about and what disagree with him about. And because a big part of Ron Paul is the idea of supporting the constitution, you might want to re-read (or read for the first time) and study the United States Constitution.
2. Register to vote as a Republican. In order to vote for him in the primaries, if you haven't already done so, and if you don't live in an open primary state, you'll need to register to vote as a Republican. It's usually just a matter of printing out, signing, and mailing a form to your county's local Republican office. Relatively few people vote in the primaries, usually less than a tenth of the population, so you could have a larger impact than you'd suppose.
3. Talk to your friends, family, and acquaintances about Ron Paul. People are more likely to be convinced by someone they know rather than a random stranger, a sign, or an internet argument. I'm not saying to cold-call your 72 cousins and talk to them one by one, just, when in contact with your family and friends, don't be afraid to mention who you support, or to talk about him if the subject of conversation veers into political matters.
4. Join your local Ron Paul Meetup group. They exist in just about every area in the U.S. Remember that because his supporters come from all walks of life, there will probably be a lot of disagreement and differences between people in these groups: 9/11 truthers, fundamentalist Christians, anti-immigration activists, Objectivists, and so on. If you do attend these meet-up groups, try to work beyond such differences, focus on commonalities. There are other places where you can keep in contact with other Ron Paul supporters besides Meetup: there's an IRC channel (#ronpaul on irc.freenode.net, irc.servercentral.net, or us.undernet.org), there's forums, and of course this LiveJournal community.
5. Create something! Use your own special skills and talents, your field of expertise. For instance, I'm an independent games developer, so I've been planning to create a small Ron Paul game. If you're a writer, a visual artist, a musician -- write something about him, draw something about him, or write a song about him! Think about what distinguishes you from other people, and use that talent creatively. This is probably the most important one of these steps if you can do it, with the most potential, because you'd be doing something for him that only you, and nobody else, could have done.
6. Donate to his campaign. The amount you can contribute to his campaign is limited to $2300.00 by law. Not all of us can afford that maximum amount, but most of us can afford $50.00 to $100.00 or so. I sold a few unneeded items (old Playstation games) on eBay and donated the profits, so if you're familiar with eBay, try that! You might may not have extra money lying around, but most people have extra unneeded items lying around.
7. Promote Ron Paul using yourself and your property. It's been shown that yard signs are far more effective than signs on public property, so if you own your own property, especially one where people often drive past it, consider putting up a sign on it. Homemade signs work even better than stock campaign signs, especially if you have any artistic ability, and it shows you care personally (here are some example slogans). If you own a car, bumper stickers are also easy to do. Ron Paul t-shirts are another easy and effective method, especially if you go out in public often, and there's a great variety of designs. And it's easy to add a link to his site in your e-mail or forum signature.
8. Write a letter to the editor for your local newspaper. It's a fairly easy way to help the campaign, and most of the time the letters can be emailed. And if published they usually have an audience of a few thousand -- and because most readers of newspapers are older, you'd be reaching an audience that probably never heard Ron Paul's name. Generally people who use the internet for news don't read newspapers, and vice-versa. They should be well-written though, editors usually won't publish the ones with poor writing or which aren't interesting to read. But there's often very little competition, I got one published pretty easily. You can find newspapers near you using this list.
9. Promote Ron Paul online. This is the least important of these, but it still has a tiny benefit and is relatively easy: if you're on MySpace, friend his his MySpace; likewise for Facebook, YouTube, and other social networking sites. And you can browse recent blog posts mentioning Ron Paul and engage people in dialogue through comments. But my advice is not to get too caught up in that sort of thing, because, even though it seems like a little effort, there's really not that effective in comparison to the other methods mentioned above, because there are already tens of thousands of other people doing these sorts of things. But stats like these admittedly sometimes raise his exposure.