Keep button 1 pressed while pushing the START button to continue from a previous game, as well as select which level.

Enter the service mode and select sound 19. Press the service switch to view the grid and enter U, U, U, U, D, R, R, L, L, L, L, L, L, START P2.

Find a hidden flag along the river.

Reach Round 3 (view the first cutscene)

To get better upgrades (Galaga Arrangement) let your ship get stolen, then let another steal your ship. Kill one, get the power-up, then kill the other bug. You will then get an ultra upgrade. Try using different bugs.

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.

On most revisions of Galaga, there's a trick that stops enemies from firing at your ship. In Stage 1, kill all aliens except the two bees in the bottom-left corner. Allow the bees to dive down and shoot at you, while dodging their shots. After around 15 minutes or so, the bees will continue to drop down, but they won't fire. Let them drop down another 5 or 6 times, then kill them. After all this, enemies will not shoot at you for the rest of the game.