Take care to zoom in when setting a waypoint too, to make sure it's on the road you want to travel on. It might sound obvious, but it's really easy to drop it on a nearby road when you're zoomed out too far, with the net result that you end up taking an unexpected detour at some point. Not that that's ever happened to me of course.
I agree with the other posters - Tyre is an excellent piece of software for route planning (and playback of routes captured with GPS logging applications). There's a lot of functionality in it, which can be a bit daunting at first, but a lot of it isn't needed for basic route planning. I particularly like the way you can switch between Tyre and Google Maps when researching a suitable route.