If I am zoomed out and add a waypoint in a route, the waypoint often gets added in the middle of nowhere or on a minor road that I cant see from the zoomed out view. When I add a waypoint always add it to the road by default.
Also place the waypoint on the side of the road in which I am travelling. This may seem silly, but if you dont do this and dont notice then your sat nav keeps telling you to turn around when possible even though you have passed the waypoint, this is because it wants you to hit it on the other side of the road. Its especially annoying on motorways but is still an issue on minor roads.