1) Cover the terminals of the battery with a ziploc (probably more for piece of mind)
2) Lightly spray down the bay
3) Coat the bay in Simple Green and let it sit
4) Use a soft bristle scrub brush to loosen or remove easy to reach spots
5) Spray down the bay lightly again
6) Air/sun dry and then apply a vinyl/plastic cleaner (Meguiar's #41) or similar interior/dash type cleaner to all plastic, tanks, hoses, covers, etc.
7) lightly apply wax to the edges around the hood and engine bay. This goes a long way to keep water and debri from building up over time, allows a smooth surface for it to flow down the proper drain paths.
This is for a pretty harsh bay, but once this is done it's a pretty simple process that would only include steps 6-7 to keep it maintained. I clean out my bay twice a year (spring/fall) when I am doing my big season checkups just before summer and again before winter.
My Matrix when I got it
Of course the GLI didn't need such work cause it was my first new car, so it should be easier to keep clean.