This page shows my technique for 'Porcupining' and then cubing a mango. You will need a mango, a cutting-surface, a slightly-serrated knife (serrations are important when cutting mangos!) and a strong will to prevent you from shoving the delicious fruit directly into your mouth and forgoing the preparation.
Mangos have a rather flat seed which runs throughout most of the fruit. You can't slice through most of the fruit, so you have to work around it.
The mango will be slightly wider in one direction; this is the axis that the seed runs along. Place the mango on your cutting surface with the tall side up and down. (So the flat seed is standing on edge, inside, with pure mango-meat on either side.)
After slicing off one side, turn the mango around and slice off the other. (The white-ish spot in the center of that far picture shows that I was right next to the seed.)
Continue with parallel cuts out to the side.
When that's done, flick it around 90° and repeat the same process, making your mango into a nice grid (still held together by the skin).
(This is a great way to serve Mango by itself; it's easy to hold and bite off pieces without getting your hands or chin juicy. Plus, it looks both artsy and cute...who can resist that?)
As it is, I was cutting small cubes for a Mango salsa, and the tiny grid I made yielded tall columns. So at this point, slide the knife across a few rows at the right height to make cubes.
Put your knife point through the remainder, at the edge of the skin, right next to the stem. Rotate the mango with your other hand (leaving the knife stuck in place) to easily shave off the belt of skin.
Now you have a luscious bit sitting there on the counter. You could shave off the bits and nuggets and chop them up, but frankly...this is your reward. Pick that bit up and sloppily scrape off all the good stuff with your teeth. You've earned it :)
(The last shot is cubes of mango and red pepper in preparation for the rest of the salsa.)