Get at least three written estimates, especially if it's going to be a big job. Make sure the estimates are based on the same set of specifications, materials and scope of work. Otherwise, it's difficult to compare estimates and determine which one offers the best deal.
Be sure the estimates are detailed and specific, and cover issues such as the start date, the cost of any change orders, the expected completion date, who else besides the contractor you hired will be working on the property, the type of access they will need to your property, who is responsible for clean up and when and how much of a down payment is required.
Don't always go with the low bidder. It may be worth paying more for a contractor who has greater experience, will use higher quality materials, will do a better job, and keep to a timeline.
Be wary of an extremely low bid. It may be an indication that the contractor made a mistake in preparing the bid, is not including all the work quoted by the other bidders, or does not really intend to complete the job as set forth in the estimate.