Looking for recommendations - roofers

Off and on over the years I have sworn to get my roof replaced and have received recommendations for reliable roofers. Still haven't gotten the roof replaced, and have lost the list of recommendations. *sigh* There's still time to get it done this year! So I'm asking again for recommendations, along with any tips you might have to offer about the whole process of getting bids on roofs, how much (if any) to pay up front, what to ask the contractors, etc.