One way is to do this: (Hopefully it's the same with your Storm/I have a BB Curve)
Go to Options>AutoText>BB Menu Key>New
Under where it says Replace: type in zz or any keys you want to use as a shortcut for your signature that aren't being used in autotext
Then under With: type in what you want for a signature
Then press BB Menu Key>Save
Go ahead and type an email, pin message, sms, mms, tweet, FB, or whatever and at the end of your message hit the letters you assigned as a shortcut zz then the space bar and voila your signature will appear. Hopefully this helps.