Check that contractors carry liability insurance and worker's compensation. It's even better if they are members of one or more of our professional associations which will help regulate industry standards.
Go see a contractor's work. In fact, see work in progress, or else a contractor will show you only finished gems and not how you can expect a project to go.
Some companies quote low but nail you for extras while those with higher quotes may actually be more honest in spelling everything out from the start.
There are always additional expenses along the way but the final price shouldn't be that far from the budget being set out.
It's true of construction people too. All the good ones are taken, so don't expect to pick up the phone and have someone start next week. If they're any good, they're likely booked solid.
The more you know what you want and the better you communicate that to the contractor at the inception of a renovation project, the smoother things can go. Making up your mind or changing your mind along the way, only eats up time and drives up the final price.
Expect weather to affect the project's timeliness and maybe even create challenges and setbacks.
People hire a contractor because they don't have the time or skills to do the project themselves, so let the professionals do what they know how to do.
Set a realistic plan. Realize that projects are fluid entities that change and there are always labour pains. Roll with it!