The Maillard reaction is the secret king of cookery, the little bit of magic that turns a slab of meat into a nicely browned steak or a raw onion into a caramelized delight. We call it "browning," and it's a reaction between proteins and sugars in the presence of heat that creates the tasty flavor compounds.
I'm equal parts lazy and impatient, and always give up before my onions are nice and browned. There are a few things you can do to assist the reaction, though.
You've probably put an egg wash on a bread at some point in your life - that's a way of increasing the available protein, giving the Maillard reaction a little bit more to work with. It isn't very useful for onions unless you're making an omelette, though.
BUT! According to an age-old post on Khymos, it turns out there's another way to help out the Maillard reaction: increasing the pH (a.k.a. making it less acidic/more basic). Watch what happens when you add a little baking soda - a weak base - to some frying onions:
Incredible! The power of science will save you minutes every day! Read more over at Khymos.
[SCIENTIFIC PEDANTRY NOTE: People always have a hell of a time distinguishing between caramelization and the Maillard reaction. Maillard is amino acids (proteins) + sugars, while caramelization is sugars only. Both are accelerated by increasing the pH, so either way baking soda helps. Onions have ~2.5x as much sugar as they do protein, but I'll be damned if I know what that means in regard to Maillard vs. caramelization.
While we're at it, here's a great PDF from a class at Ohio state on different types of browning.]