Paperwork. Its seriously the most important thing.
From Scope of work to final release of lien, paperwork can make or break almost every aspect of your project.
Initial plans on a napkin can give you the outline, but the finite details is what really insures your success and profitability of a project.
Making sure your scope of work (the line item list of work to be done) is complete is the only way to make sure your materials list and labor cost is accurate. Its the only way to make sure your timeline (holding cost) is appropriate for your project.
I have found that detailing out what I need for my project leads to additional information being added I had initially forgotten. "Write down your goals" is a super valuable trick to insure your success. Write down what you want to accomplish, how, and when. Then follow that plan, and if you did it right your won't need to make major changes along the way.
Providing this information to your lender and trades will make the whole process easier. It insures you have LESS problems along the way, and in the end we can only attempt to solve as many issues before we are financially invested as possible.
Paperwork from bidders and hired trades are incredibly valuable too. Making sure you have ALL the details of what you expect in writing. If something unforeseen changes along the way then write that down and agree to the solution, in writing, before moving forward. Make sure you get warranty, bond, and releases of lien from your trades too. I suggest a "contingent release of lien", upfront. This states that the lien rights of that trade are released in conjunction with a cleared check. You don't want to have to chase someone for the paperwork you need, or worse; have them try to collect for what they have already been paid.
Trust, but verify. People memories can be faulty at times. Having your plan and agreements in writing, upfront, can solve a lot of problems before they ever arise.