Wait until one of the top ships(the green ones that take two shots to kill) comes down and fires off a tractor beam(the beam that can capture your ship). Let it capture your ship. Now, wait until it swoops down again with your ship behind it. Simply kill the head ship(the green one), and it will lose your ship. Your ship will then lock on to the side of your current ship, giving you two ships to play with at the same time! Now, be careful. Not only will both ships be firing at you after your ship gets captured, but if you accidently shoot your captured ship, you'll destroy it. Also note, this will cost you one life, but will definately make the Challenging Stages, and perhaps the game itself, a little easier to play.

Note: This works in all non-bootleg/hack versions of Galaga, as well as most of the sequels and Arranged Mode in Namco Classic Collection Vol. 1.