My best tip is to MEASURE, MEASURE, MEASURE. This means that every time you add another block, or another row, measure horizontally across the top and bottom of the quilt so far, and make sure they are the same. Then measure vertically down the sides of the quilt, and make sure they are the same. Do this all the way, as you add to your quilt top, and at the end, your quilt should be square i.e. the width top and bottom are the same, and the length of the two sides are the same.
My 2nd tip is not just to measure the top and sides, but to measure through the centre of the quilt, both horizontally and vertically, to see if it is the same as your top and side measurements taken previously. This helps to make sure that, as you add more rows to your quilt, it does not become narrower, or wider overall. This creates fullness, which is very difficult to quilt out.