Use the same tape measure for all your measurements.
Keep the tape square to your work, measuring something at an angle can create inaccurate readings.
Look at your tape straight when reading a measurement graduation.
Use the smallest unit your tape has to be accurate, most tape measures have units to 1/16 or 1/32 of an inch.
Double check your measurements, it's the old adage measure twice, cut once that will save you money and aggravation.
Use help when measuring something long to keep the tape straight, sag in the tape will create inaccurate readings.