Experiment With Gmail Labs
Gmail Labs options are a great way to try various experimental Gmail features. First choose Settings, Labs
and then click enable
on the ones you'd like to activate.
I especially like Quick Links, which adds a box containing a list of bookmarked links in Gmail to your sidebar. It's a great feature if you routinely run complicated searches, as it allows you to bookmark searches, individual messages, and labels.
Two other options I recommend are Undo Send, which gives you a few seconds of lag time between when you hit 'send' and when Gmail actually does the sending (giving you a grace period for correcting belatedly discovered mistakes), and Mark Unread From Here, which enables you to mark a conversation unread from a certain point on (great for long message threads).