Attachments can take up a lot of space. If you don't need to store them in your mailbox, don't. Instead, use the Save Attachment command on the File menu and save the attachments to some convenient location on your hard disk. Once you've saved the attachment, you can delete the message (if you don't need it anymore), or you can right-click the attachment icon and click Remove to keep the message but remove the attachment.
Before you start saving attachments to your hard disk, make sure you have a location in mind that you'll remember later. For example, if the attachment is a photo, you may want to save it in the My Pictures subfolder of My Documents. Also, when you save the attachment, make sure you give it a name that will help you remember its contents and context.