Tease with Social Media, Launch with Email
The most powerful platform for launching a marketing campaign is by far your email list. You'll be able to reach far more people through your email list than through any other avenue.
Use social media to tease people about upcoming launches. For example, drop hints about what might be in a new product, or tell people that an awesome contest is coming up, without actually letting on what the contest is.
Once it's actually time to launch, use your email list to get things kicked off.
Enroll Cheerleaders and Affiliates Early
Reach out to people who can help you early on. Talk to people who could retweet you once you're ready to launch. Reach out to potential affiliates as early as possible.
Cultivate relationships with people in your industry before you need it. When you're ready to launch a campaign, you should have allies across all kinds of media. You should have people retweeting for you, affiliates mailing promotions and bloggers writing reviews.
Have Two-Way Communication
Build a two-way communication channel with your audience. Use social media and use blog comments to get feedback from your audience. Having open communication also lets your users feed off of each other's excitement.
If you were just using email marketing, people can't talk to each other about your launch. They can't share what they think is coming up, or ask questions of each other. Make sure you have at least one channel of communication that allows your audience to talk to you or to each other.
Using SMS and Direct Mail
SMS and direct mail are two forms of media that are seldom discussed.
For each promotion, product launch or contest, send just one or two SMS messages. Don't overload your audience by sending too many texts. Send your text when it's most timely.
Direct mail is a fantastic avenue for reaching previous customers. It's only really valid for product or event related campaigns, since it's quite costly. You have to be able to make money by making sales to recoup your costs.
These two tactics aren't mandatory, but can really make a big difference when used properly.
During Down Times...
When you're not doing a campaign, do as much as you can to cross-promote your different kinds of media. Use your email list to grow your social media following. Use social media to boost your blog traffic. Use your blog to bring on more email followers. So on and so forth. This makes launching campaigns much more powerful once you get around to it.