Here is another one you can file in the ‘Duh’ category, yet seems to be foreign to many people: when emailing a large file, zip it first.

Whenever you email, for example, a 1 megabyte Excel file, not only does it send a 1 MB email to each recipient, but also keeps a 1 MB copy in your sent items as well. Considering Excel files (in this example) typically compress very well, you could instead be sending a 250 kilobyte file instead. This saves both you and the recipient space and also cuts down (granted almost insignificantly) on bandwidth use. Additionally, compressing the file can help get your message under your recipient’s email size limit.

I know many people are spoiled by the Windows shortcut of right click > send file, but instead consider using 7-Zip’s compress and email function to accomplish the same thing.