I've read an article about what can happen if your Inbox grows to a decent size and your Pocket PC starts suffering of "lack" of memory. Their suggestion is great, but I also found an article on e-mail management worth a read.
According to the post on Pocket PC Addict, when you use Activesync to transfer e-mails to a Pocket PC, the option to transfer the "last x days" actually does not remove old e-mails from your device. The e-mails stay there, until they're manually deleted from your Pocket PC - even if you delete the e-mail from your Outlook INBOX! Remember, ActiveSync will sync the last "x" days, and if the delete e-mail is out of this range it will not be removed from the Pocket PC simply because ActiveSync will not select it.
As a personal routine I have my ActiveSync set to synchronise all contents in my Outlook Inbox and some sub-folders. To reduce the size of my Inbox I have folders under my Inbox divided by areas - Geekzone, Home, Work, Competitions. I also have rules set so all e-mails arriving in my Inbox are then moved to the appropriate folder.
Once I read an e-mail and actioned, I move it to an archive folder - or delete it. This helps organising and keeping things clean.
A great help is available from the goodexperience site. There's a free 35 page report on how to manage e-mails (the download link below). It's a great resource. You'll find that the best action is... action! Once you have read an e-mail, decide what needs to be done. Write it down on a to-do list (Pocket PC and Palm are great for these ). Then either archive the e-mail or delete it. Simple. Easy. No more huge inbox folders.