For running dado rails through a hallway, or making a wallpaper border level round the room, an excellent tool to have in your kit is a chalk line. Simply mark a point on the wall at one end of the room by measuring from the ceiling or floor, then do the same at the other. Hold the chalk line on both marks, lift it and let it go. "Ping".
Also use this on horizontal surfaces to mark the floor when setting out for laying floor tiles or on the joists when laying floorboards. A chalk line can be used in almost any situation where a dead straight line is required.
If you are on your own, then knock in a small tack or nail on your first mark and loop the end of the chalk line over it. Hold it tight on the second mark and ping.