Pre-shrink fabric (whatever your preferred method) prior to cutting and do not use steam when pressing unfinished block seams.
Quick story: one of my first quilts had used very small flying geese in the sashing and I used an assortment of fabrics for the scrappy look I desired but when I went to assemble the sashing, discovered that many blocks had shrunk anywhere from 1/4" to 1/2". I learned the lesson the hard way and started using fabric sizing or spray starch to pre-shrink and stabilize the fabric before cutting.