I have both my personal and professional e-mail forwarded to a single Gmail account for convenience. One complication this causes is the need to use different signatures for correspondence from a single account. In the past, I've used the Blank Canvas Gmail Signatures extension in Firefox, but that has two drawbacks:
1. It has to be installed and the signatures configured separately on each computer I use.
2. It only works in Firefox.
Credit goes to an entry at thenextweb.com for pointing out a better way. Just use the Gmail labs feature "canned responses". Save each of your signatures as a canned response, and then you can add it automatically when composing messages. This works in every browser and only needs to be set up once. Contrary to what it says on thenextweb.com, you can include html in your signatures when using this method.
Something to watch out for: canned responses are actually saved as messages in your drafts folder. They are hidden in the usual Gmail web view, but are visible in basic HTML mode or if you access mail through your phone. Don't delete them.