In addition to familiar "]" and "[" keyboard shortcuts for archiving and going to next/previous conversation. Google's offering a new Lab feature called "Auto-advance," which automatically opens the next conversation after you archive/delete/mute the one you are on.
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.