I love gmail as much as the next person, but I've always been a little perturbed that I didn't know how to compose an e-mail in a separate window. It's a little thing, I'll admit, but composing e-mails on the one window makes it harder to reference other e-mails, or in my case, focus.
How do you make life easier? Just hold down the Shift key when clicking "compose" or "reply" to pop a separate window, and search and browse your existing mail in the parent window while you write. You may need to enable popup windows from the site if you get an alert/nothing happens.