The short answer is: Be awesome.
If your software really addresses a pain point and addresses it well, people will come to it on their own (assuming a reasonable amount of promotion on your part) via SourceForge/GitHub/etc., Google, and word of mouth. If you attract a critical mass of people who need what your software does and need it to have new features, collaborators will come.
Also: Stay active. If you haven't updated your code in 6 months, or if your page on SourceForge says your software has been in Alpha for a year, a lot of people are going to ignore it and move on to the next search result. Get the software to where it's ready for others to use, and maintain it.
The thing about Open Source is that it really has to grow organically - nobody's getting paid, nobody's under the threat of being fired - people have to care about your software for it to grow beyond what you can do. You're never going to get collaborators (or anyway, good ones) by soliciting for them - only by making something that a good programmer would find interesting to work with.