First, I would make sure you have a good chalk line with a decent sting on it. Once the string starts to get "fuzzy" it no longer makes crisp lines and should be replaced. Also, when you are squaring off with a line, I would draw the perpendicular line extending outward on BOTH sides of the line. Finally, I would do a 3,4,5 check to see if it is square (3 feet from the intersection on one line, 4 feet from the intersection on the other line, and 5 feet between the 3 and 4 foot marks).