From running dado rails through a hallway to making a wallpaper border level round the room, an excellent tool to have in your kit is a chalk line.
How to Use a Chalk Line
Simply mark a point on the wall at the height that you want your line to be at, at one end of the room. You can either mark the first point by free-hand e.g. just make the mark or you can measure it from either the ceiling or the floor. If you are using the ceiling or floor as a measuring point make sure that it is level by using a spirit level as when you come to mark your point on the wall at the other end of the room, if the point you are measuring from is off then the line you are about to create will either slope up or down.
Once you have made sure that your two points are level hold the chalk line on both marks (you may need someone to hold the line at one end), lift it and let it go "ping".
Once the line has "pinged" it will leave a nice straight chalk line for you to follow - sweet!
You can 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 away!