First, ONLY use distilled water. This will prevent from getting build-up on and in the machine.
I have found using oxy-clean (the purple bottle) VERY carefully on the coils works. Make sure you rinse with distilled water ONLY. This will help prevent excess maintenance.
You should only be using distilled water in it. If you use tap water, there will be mineral deposits from the water left behind, which will build up overtime and partially block the steam's exit... to clean away those deposits, run one cycle with 50/50 distilled water and white vinegar, that should get rid of any build up. Make sure you run one or two cycles right after with just distilled water to get the vinegar scent out and you are good to go!