Keeping Lines Straight

I've learned that rulers are not only great at making straight lines, they can help you keep your lines 'straight' when working from a list of addresses.

It's so easy to pick up a number from the line above or below the one you are working on or begin an envelope 'Mr. and Mrs.' when it should really begin 'Ms.'  This is a great little trick that has helped me eliminate that sort  of error.

Now, if I could just come up with a trick that would help me eliminate transposed letters and numbers. . .