"Thank you, but I am declining my offer to ____" . They will respond with "Thanks for letting us know early." (I did this last year, and this is how it went with everyone I declined) No one likes getting screwed over by people dragging things out until the deadline.
Seriously, don't waste time on this. The schools want to know ASAP if you're declining so they can make more offers. People who are waitlisted want to know ASAP so they might get an offer. People who want their financial offer to be bumped up want to know ASAP so they pick the right school. Just do it and move on with your life.