Provide all contractors with accurate plans or drawings that will enable them to determine the scope and cost of work.
Do not automatically accept the lowest bid.
Discuss the bids in detail with the contractor.
Sometimes a higher bid may be worth it. If the materials to be used are of higher quality. Or the work listed on the bid is more extensive (no hidden costs) it may be justified.
Beware of substantially lower bids. All the work needed may not be included setting you up for a dispute with the contractor when he tries to charge more in the end..