I've learned from dealing with some recently that I have to define exactly what I want done on paper so that there is no misunderstanding or room for them to scale back on the amount of work they plan on doing. For example if I want a tree removed I have to describe whether or not I want the roots removed, stump grounded down, etc. You don't want to have to say, "I just assumed you were going to do X, Y, and Z before you left." Consider all of the details you can think of. This can be difficult if you're not familiar with the various approaches and don't know all of the details involved (its why we hire professionals), but the more you can research and document the better.

I've learned from dealing with contractors recently that I have to define exactly what I want done so that there is no misunderstanding or room to scale back on the required work. 

For example, if I want a tree removed I have to describe whether or not I want the roots removed, stump grounded down, etc. You don't want to have to say, "I just assumed you were going to do X, Y, and Z." Consider all of the details you can think of. This can be difficult if you're not familiar with the various approaches and don't know all of the details involved in a job (it's why we hire professionals in the first place), but the more you can research and document the better.