The best tip I've seen so far is to use a lined index card to test your 1/4" for accuracy. They are so convenient to use because they can be sewn on, the lines are visible and the lines are absolutely accurate 1/4" inch apart. You have lots of chances to get the needle position right, don't have to hunt around for fabric that contrasts with whatever thread you have in your machine, and they're cheap and plentiful.