To get started with "Auto-advance" go to the Labs tab in Settings, enable it, and click the "Save changes" button. By default, "Auto-advance" will advance to the previous (older) conversation in your inbox -- perfect for people who read their newest mail first. If you usually read your oldest email first and would rather advance to the next (newer) conversation, you can change the direction from the General Settings tab.

Hopefully this will save you some time the next time you have to deal with a crowded inbox. Try it out and let us know what you think.
source: http://gmailblog.blogspot.com/2010/10/new-in-labs-auto-advance-to-next.html