Well, others have covered it, and this is only my second fixed gear, so I'm no expert but this is what worked:

1) Chainline, chainline, chainline - that's #1 because of the number of moving parts, and the amount of stress they are under, they will be noisy as he(double hockeysticks) if off even by 2mm
2) Spend enough on the components - you don't have to go Phil, but cheaping out will get you noisy stuff
3) Lubricate the chain regularly - I like white lightning

HTH