How to establish a relationship with contractors and rehabbers

[Brandon Gamblin]

How do you go about establishing a relationship with a contractor? Is it best to do this when you've already bought a property going into rehab, or before you have anything under contract? What is the extent of an investor’s due diligence when looking to work with a contractor? 

Below are some things I've considered, but I'd rather get other's more qualified opinions:
1. Do I ask the contractor if they charge a fee for rehab estimates?

2. Are there any documents that should be signed to establish the relationship?

3. What things should be discussed upfront to promote and protect both parties interests?

4. Should this be a long term relationship or short term relationship?

What other questions are there that I should be asking? What am I not considering?