Most concrete floors and cement slabs crack because of poor compacting of the subgrade. Most concrete contractors take the time and effort to do this. But in case you get one of those other guys it wouldn't hurt to make sure, compacting the subgrade is one of your concerns when you shop for a cement contractor. Your subgrade should be compacted in no more than six inch lifts. For every six inches of dirt thrown in to an area the contractor should compact it.
If your house is being built in a wet or swampy area you want to put a little extra cash into the preparation. Have your cement contractor run some perforated tile under the cement floor through the footings. These will run into drainage pits filled with pea gravel. Then cover the subgrade of your concrete floor with four inches of pea gravel. This will keep the pressure of the water from pushing its way through the basement floor. Having too much water in your cement mix will give cracking in your floor. Concrete should be poured with a consistent slump. This is the consistency of the concrete off the end of the chute. You want to have the concrete a little stiff but workable. What you don't want to see is the concrete coming out of the chute looking like soup. The more water in the mix, the more the concrete will shrink and crack. Cement slabs can shrink as much as a half inch in one hundred feet as the excess water evaporates from the poured slab. Excess water will greatly reduce the strength of the concrete.
Your concrete will also crack if it dries to fast. As your cement floor dries, spray a light mist of water on the top so it won't dry rapidly. Another thing you can do to prevent rapid drying is cover the cement slab up with plastic or a light tarp. If the basement floor is being poured with the floor already installed, it probably will dry without any problems.Have your contractor put control joints in your concrete floor as its being finished or cut in while it is still green. All concrete contractors have had a floor get a crack here or there. Sometimes cracks are unpreventable, that's why control joints are so important. The control joint creates a stress relief in the cement and prevents the floor finish from cracking.