That's a very good question though I never got opportunity doing myself!
What I guess is to pick up beta or any development stage version projects (for example apache site) and start working on it. If you progress on something, update the source code though I'm not sure of how this is done. You can better email administrators or relevant mail groups and get to know. Sorry, my answer may not be helpful.
(OCEEJBD6, SCWCD5, SCDJWS, SCJP1.4 and Oracle SQL 1Z0-051)