Are you doing extract kits? If you are too high on an extract kit, you just add more water in the top-off step after cooling. If you are too low, you added too much water. The efficiency of extract is already set (i.e., the sugars have already been extracted), so short of messing up the temperatures or not actually getting all of the extract out of the container, it is hard to mess up OG.
A common mistake new brewers will make is trying to ALWAYS top off water up to 5 gallons, when you should actually top off slowly until you reach your OG, then stop, even if it's more or less than the 5 gallon target. Top off to your target OG, not your target volume.