The measuring tape should always be parallel to the floor. If you measure on an angle you will get an incorrect number
Remember to breathe. After placing the measuring tape around you take a full breath in and out. We tend to hold our breath when we get measured which doesn't give an accurate number, so take a breath and get it right.
When taking a measurement remember to not pull too tight, the tape shouldn't be slack but it also shouldn't be tight enough to cut into you.