Short answer: Be healthy.
If your software really addresses a sore point and solves it well, people will come to it on their own (subject to reasonable promotion on your part) through SourceForge / GitHub / etc., Google and by 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 after 6 months, or if your SourceForge page says your software has been in Alpha for a year, many people will ignore it and move on to the next search result. Get the software where it is ready for others, and support it.
The thing about Open Source is that it really has to grow organically - no one gets paid, none of them can be fired - people have to take care of your software so that it exceeds what you can do. You will never attract collaborators (or, in any case, good ones), asking for them - only by creating what a good programmer will find interesting to work with.
bradheintz
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