Try sorting your mail by ‘From’ or ‘Sender’. Sorting by ‘From’ will let you quickly identify entire blocks of mail from individual senders that can be quickly deleted.
For instance, you get a newsletter once or twice a week. You may keep them around for a while to read later. So over time it just looks like an occasional message, but now that you sort by ‘From’ you’ll see all 50 copies of that message in your inbox and can quickly delete them all. Do the same thing with ads and old discussions and you can quickly take care of a big chunk of your old mail without having to labor through the decision making process of paging through every message in your inbox.
Another nice thing about this method is that it’s easy to do a little bit at time. It’s easy to see where you stopped and continue on so you don’t have to feel like you have to deal with your entire mailbox in one shot.
Of course this won’t take care of everything, but it should clean out a lot of the clutter and make the rest of the mail cleaning process a bit simpler.