Get something nice and something that you will wear.

I get to take classes for work about fall protection.

Wear it snug but not too tight 2 fingers should just be able to get between the straps.

Make your anchor point as tall as you can, will help prevent you from generating more energy in your free fall till the fall arrest system starts to work. Also can prevent you from hitting the ground if your not too far up in the tree.

Invest in some trauma relief straps if the harness isn't equip with them. ($10 or so) If you fall and can't get down you have about 15 minutes till bad things start happening with the blood pooling in your legs. If you do fall and hang for a little bit, don't take off your harness right away. Allow some time for your blood to start flowing normal again or you can overwhelm your heart with non oxygenated blood.

Wear your leg straps, failure to do so and you most likely will slip right out of the harness in the event of a fall.