Save important e-mail as a text document - with the message open, go to File > Save As. In the "Saved message format" drop-down choose "plain text." Then in the "Current directory" box, choose a location to save the file.
Delete unnecessary Sent Items - including appointments, reminder notes, etc. from your Sent Items folder. They will also be deleted from your calendar.
Delete unwanted mail
Archive mail - Select a message, Actions > Move to Archive. This is not available in web access.