Ok, this tip is probably most useful for those that are using two monitors. Of course, if you're not using two monitors, I can't encourage you enough to add that second screen.
A lot of people will add an attachment to an email by composing the email, hitting the attachment button, then locating the attachment in the file directory.
However, if you have a copy of the file you're going to be attaching on your desktop or open in file explorer, you can just drag the file on to the mail tab in Outlook.
By doing so, you will open up a new email with the document already attached.
Alternatively, you can right click on the file and choose "send to" and "mail recipient", which has the same effect.
The big point: there's usually more than one way to accomplish a given task. Find the one that works best for you.