Biggest thing with tape measures is making sure that you are parallel with the edge you are measuring. It is amazing how having the tape at a slight angle can throw off the measurement, especially over a longer distance.
The clip end also has to move, it slides in for inside measurements, out for outside measurements to compensate for the width of the clip. However, if the clip is bent, or the tape has been abused, those holes might be too large, and you're going to introduce some error. Treat your tapes nice, and they'll be reliable much longer.
I use tape measures for 'coarse' measurements and if I need something more exact I use my scales. This works great for smaller parts, which are often hard to measure and mark with a tape, and for machine setup. I also use a 6 inch dial caliper if I want really accurate measurements of small stuff, and brass guage blocks like the router guys use for tool setup. A decent dial caliper is often under $20. Of course, my scales only go up to 2 feet, so if I'm over that I have to either improvise, transfer the measurements to story sticks, or trust a tape.
If I need an accurate measure from a tape, I usually start at the 10 inch mark and work from there. That removes the end from the equation, and hopefully some of the error. Also, to improve accuracy using only a tape, stick with one tape measure for the entire job. I've found in the past that some of my tapes do not agree exactly, and if your using one at the bench to lay out your parts, and one over by the table saw to check the parts after the cut, you could be chasing your tail.
As big a deal as reading a tape is putting your pencil mark on the board. A hastily drawn mark can be confusing as soon as you pull the tape away. I like to use 5mm pencils and make a 'v' at the mark, instead of just a dash.