This one comes straight from the geniuses over at Lifehacker and one of those tips everyone needs to know.

When you are reading a message in gmail, press the # key (Shift+3) and that message will instantly move to Trash. Alternatively, select one or more email messages in the gmail list view, press the same # hash key and all the selected gmail conversations will be deleted in one batch.

Didn't that just change your life? If you want more gmail shortcuts and tips check out the keyboard shortcuts list online.