I've found honesty is always the best solution. It is painful in the short term but works in the long run. Take mom and dad to lunch or sit them down for coffee at home and just have an honest conversation. I've had to tell my parents several things they'd rather not hear--moving in with my boyfriend (across the country no less), getting a tattoo--and I just told them straight out. While they were upset at first, they always calmed down later and realized they loved me anyway.
What about first telling them you're considering it? That will give them a chance to get used to it, even to offer their opinions and advice. Even if you don't take they advice, they'll feel better because they've been involved.
Personally, I wrote my mum a long letter - the longest I'd ever written (I was abroad at the time), then I introduced her to him at the earliest available opportunity. She fell for him and spent the next twelve years - and counting - taking his side in all disagreements!
don't tell them straight out and let them kow you minds made up say i might be he's asked me and i really want to let them still have a say in ti and make them feel like they still have some say.