Many projects have pages explaining how to contribute; the Twisted networking library, for example, has a great list of suggestions. But if none of those is something that can be done right now, a potential contributor might leave the page and forget to come back.
To make it easier to express interest, we just added a new button to our project pages. This is what it looks like:
With one click, you can appear in a list of people standing ready to contribute. Now the project knows where to find you. Make sure your profile says what you like doing, and most importantly, leave contact information!
For current contributors, your project page will list people willing to help. You can read their profiles and see what they want to do. Maybe one of them lives nearby; try organizing a two-person bug jam! Once you extend them a personal invitation, they might help out.
To try this out, go to https://openhatch.org/+projects/ and find a project you like.