In addition to the post above, I would suggest making sure all walls are dust free, particularly unpainted drywall. If there is much dust at all on the wall the primer will not adhere well and the paint can get erasers sized bubbles in it, usually when you apply the next coat. These bubbles will get larger and more numerous when repairs are attempted so it's important to get this step right the first time. Also, use primer on all walls (even old paint) to make sure there is a good bonding surface and since you have no idea whether the old paint is oil base or latex. You can get the primer tinted to closely match your new color which reduces the need for additional coats of paint.