Measuring by yourself can be a drag. Here are three gadgets to help you out.
If you are working with metal studs, you can now find a tape measurer that has magnetic on the hook. That won't slip and you can even just put it on the surface of the stud and measure from there.
To get the hook on your regular tape measure to grab better you can add on a little thing called "Grip Tip" that looks a bit like a soda pop bottle top. That gives you a larger hook with a grip to boot. This one does change the measurement by 1/8th of an inch, but you can simply take that into account when marking out your measurements.
The third tape I showed was one of the new electronic tape measurers, this one from Zircon. There are electronic tapes that are more accurate than this one, but for several hundred dollars more. This one does a great job of estimating paint, tiles, rugs or even siding. Since all the inexpensive electronic tape measurers like this one actually measure an average of everything that falls into a cone of sound that comes out of the tape rather than actually measuring to an exact dot, you can be confused as to just what it is measuring. With this one, they provide a laser ring of dots as well as a centring dot that makes you aware of exactly what is being measured. No confusion and operation so simple that you don't need an instruction booklet.