Apples to Apples

Make sure each contractor is quoting the exact same scope of work. One bid might be high because you mentioned to them in the walk-through that the trim needed to be painted but didn't mention it to the other contractors.  

Make sure you ask your contractors to specify what products they use. If you'd like to use a specific manufacturer, make sure you let each contractor know so they can bid accordingly. If one contractor submits a bid using low end materials and the other one submits their's using mid to upper end materials then their final numbers may look very different.

Ask Why

Once the final bids are in you may notice some of them are high while others are low. Ask why. There may be underlying reasons these bids are so different. Does one of them have a crew who can get the job completed in a few days while the other is one guy working from the back of his van? If you have a strict deadline the lowest bid may not be the best way to go here. 

No matter who you choose to do the work, make sure you've done yours.