Outlook allows up to 2 active signatures for each email account on your system depending on if it's a NEW email or a REPLY email. For instance, I have 5 active email accounts (I know, I'm neurotic but I can't help it!) and I have about 7 signatures set up, but can only use 2 per account.
Well, all that changes with Outlook 2007. Not only can you still have multiple signatures, but you can use them on an "as needed" basis.
First Tip: set up your signatures.
Tools > Options > Mail Format tab > Signatures.
Click New, choose which account to apply it to, and start typing
Second Tip: Click New Mail Message and notice on the Message ribbon there is a "Signature" button. Click it and choose the signature you want to use.
Steroid Tip: If you don't need a formal signature for coworkers or colleagues but it is set to automatically show up, just right-click the signature in the email and then click the other signature to use. This way you don't have to click the "Signature" button on the Message ribbon.