There's no easy way (through cell phone companies) to use multiple phone numbers for the same cell phone. I know for a fact Verizon Wireless doesn't offer vanity numbers, but I'm not sure about the other providers.
Services like GrandCentral (http://www.grandcentral.com/) allow you to get another phone number and forward calls to your phone and "mask" outgoing calls so it appears to have come from your GrandCentral number instead of your cell phone number. It's only in beta though and it's difficult to get an account right now.
There's also Skype. You can get an "Online Number" from them. http://www.skype.com/allfeatures/onlinenumber/ -- not an expert on this, but there should be some way to do call forwarding to your cell phone, but you might need a subscription.