Drop and Roll
It's easy for most of us to imagine the finished paint job of a new house, but a little more difficult to foresee all the things that could potentially go very wrong in the process. That's why it's important to take the time to prep accordingly for the large amounts of permanent pigment you'll be flinging around.
That means doing a little more than putting on your grungy jeans and a baseball cap. Don't assume that you're careful enough to avoid the bushes and grass around the house; cover a good-sized area with tarps or drop cloths around your painting site.
And for goodness' sake, don't even pretend your hand is steady enough to create the perfectly straight line around your trim. Tape up windows, trim and doors to ensure you don't have splotches of paint migrating where they shouldn't.