What is the best way to avoid computer spillage?

The only surefire way is to just simply not have a drink around. Otherwise, just don't knock it over.

If you're very thirsty and can't leave your computer, then try this. If you have a table in front of you with a computer monitor and a keyboard on it, you could put your hands on the keyboard like you always do when you type with a little room between you and the keyboard so you could put your drink between your forearms, so the drink is also between you and the computer keyboard. Although this seems hardcore dangerous, you're unlikely to spill the drink this way, because the drink is right in front of you, so you wouldn't forget that you have a drink on the table.

By the way, the shorter and fatter your cup is, the less likely the cup would tip over, because it's center of gravity would be lowered.