You should be constantly looking for other ways to deliver traffic to your site such as:

Be active in forums - Participate and answer the questions.

Write articles! I know it's old and tiresome and boring - Adding fresh articles with your site related content.

Ask for help - Many times in that process there is an exchange of information that leads to a deeper and "real" relationship - leverage that for links!

Tell 'em what you think - this applies to forums, blogs, articles and email responses. Like is too short to pussyfoot around and to be taken advantage of by other people.

Use your clients! Your clients have other relationships as well, and if you perform well for them, they will refer you to their other business relationships.