It's already been planned out. A guy named Robert Zubrin (former NASA engineer, etc) has been planning this stuff out for the better part of 30 years. He published a book in the mid 90s called "The Case for Mars" in which his plan for a manned visit is described and defended. I read it, and even met him once: he's a pretty smart guy. Anyways, my point is that the entire thing has already been planned out by enthusiasts in every last technical detail whether it's a return trip or one-way-only. All of the technology exists as basically off-the-shelf parts (though as rt said, replacement parts will have to be sent from Earth).

As interesting as it would be, I agree with Isaac (or was it BlackAdder?) who said planetary colonization was a waste of resources and that we're better off moving to the asteroids and constructing entire artificial habitats and then set them rotating to use inertia as a substitute for gravity. We'd only need to create one orbital factory and bring in asteroids for raw materials to make as many habs as desired and never have to worry about those pesky gravity wells at all.