I got an offer from a school in March for a PhD program, with an acceptance deadline of April 1.  At that point I hadn't heard back from any other schools and thought I wasn't getting in anywhere else, so I accepted their offer and paid them the non-refundable deposit.  I have since gotten another offer with excellent funding and definitely no longer want to go to this school (to be honest I never really did, I was just concerned I wouldn't get in anywhere else).  I won't ask for my deposit back, as I know that's in poor taste, but how should I tastefully phrase the withdrawal e-mail in a situation like this?  I am withdrawing because I want to go somewhere else and I'm not sure if I should disclose this, or what I should say.  While I have no desire to attend this school, I also don't want to burn bridges with anyone.  Anyone else written withdrawal e-mails of this nature?  
 
Thanks in advance