If you want, you can make your "extra" address the default address in the "From:" field. That way you don't have to choose it, it's just there, every time. Making it your default address is also easy. Here's how:

1. Click Options in the upper-right corner of your Mail page and select Mail Options.
2. From the list on the left, click Accounts.
3. Click Add or edit an account.
4. Select your extra mail address from the list on the page by clicking it.
5. Click Make Default in the box on the right.

Now you don't have to select this address from the pull-down menu anymore. It's selected already!