I'm not sure if I'm understanding your question correctly but here goes:

For the clean part, make sure it's just that, clean. Make a habit of dusting and cleaning your components using compressed air cans or blowers. And some isopropyl alcohol for deeper cleaning sessions.

For the cool part, if you're talking about temperature, have lots of airflow going on in your PC, make sure all the components that need cooling get their fair supply of fresh air through proper fan placement and always provide a way for the hot air to be exhausted out of your PC.

As for "looking" cool, that's mainly subjective mate. Just go with whatever you consider aesthetically pleasing and pick your parts accordingly.