Send them about two mailings a month at most, and make sure you include some useful information - not just a bunch of ads. You can also purchase email addresses in very targeted categories. This can work very effectively - you can target people that love Sushi and send them a personalized offer. However, this type of promotion tends to be expensive, with names costing 15 to 20 cents each. The best way to try this type of marketing is to purchase a sample of the list and do a test mailing.
Some companies offer a banner ad exchange program.
You put a banner space on your site, and you get a certain number of ads on another site in return. You can even select the type of sites you want to put your ads on. This works great if you're getting a lot of traffic but don't have a huge budget. Beware of promotions and opportunities that address a large group of people outside your customer base. They tend to attract a huge audience of people that aren't interested in your product or site, and instead cost you money and time, especially if you're paying for your bandwidth.