You are on the right track...

Define very clearly the expectations. Example, if the take a tree, what are they expected to do with the limbs and left over wood? Drop and run, pile? Or are they supposed to put all tops in a single pile or multiple piles?

Define what trees are going to be harvested.
Define what trees you want them to take down (at your expence) and add to the refuse pile.
Access into and out of the timber...are they cutting paths in and out? will they grade when they leave? Seed? Or are they going to seleect cut and not make a permanant access?

Basically sit down and write everything you do NOT want to have happen and then a list of everything you WANT to have happen.