If you don't want to spend the money on a clinometer, there's an easy way to get GOOD measurements.
Get yourself a torpedo level (8 bucks or less at Lowe's). Use the 45 degree bubble. Set the level on a stick of some kind, or better yet a camera tripod. With the level at a 45 degree angle (the 45 bubble will show level), sight along the side of the level at the tree. Move backwards or forwards until you are sighted at the top of the tree. Mark where you are. Measure from there to the base of the tree. On level ground, that measurement will be the height of the tree.
The key is you are forming a triangle. The tree is one side (the elevation), a line from the tree's base to you is the other side (the base), and the line from you to the top of the tree is the third side (the hypotenuse).
As long as the angle of the hypotenuse is 45 degrees, the base and elevation are equal.