If you are breaking spokes, either the wheel is old with fatigued spokes and needs a rebuild with new spokes or it is not properly tensioned. A properly tensioned wheel should ring like a bell if you snap the rim with your finger. (note that a mounted tire will muffle the sound). It should be straight and true. Having tension equalized on the entire wheel is as important as having it straight and true. If it is not properly tensioned, it will not stay true.
Note the previous reply that mentioned that building wheels is not a money maker for bike shops. I contend that to really build a wheel right takes more time than a bike shop can afford. A knowledgeable and talented amateur who has a day job and can afford to spend a few hours on one wheel may do a much better job than you can get at most bike shops.