Most people say meditation works, some use herbs to heighten their abilities and others seem to make little pillows or bags of herbs to take with them.
I think meditation is the one most people go with, I usually just see spirits walk past, but I dont know how. Hope it helped.
If you meditate with amethyst over your third eye, this will help drastically, although you need to be very cautious while doing this as the effect can be rather intense. It is good to do this excersise only after some experience with clarvoyance beforehand.