Clean rims and pads (check them for embedded debris) also help, but if everything is set-up right it may just be the quality of your brakes.
Like SS says, OE pads are usually rubbish and you'll notice a big difference with good ones. If you don't already have cartridge type pads, switch to them: they're cheaper in the long run and you'll have a greater choice of quality inserts.
Better cables may improve things (eg Jagwire Racer), but you'll notice less difference for the money compared to good pads. I'd be inclined to buy good cables to maximise the performance of good calipers, rather than to improve the performance of cheap ones.
Failing that, you'll need to buy better quality brake calipers.