Keep your Inbox clean by archiving emails when you've dealt with them.
Label them too if you want to be super tidy. In fact use 'Move to' to label and archive simultaneously.
Not everyone has time for all this though - if you get lots of email, don't be a slave to filing them away - Priority Inbox will guide you to the unread and starred emails - the rest are history. Plus the search is so powerful in GMail that you'll always find what you need anyway.