It really varies depending on your industry.

Generally, being a social person (which does not come easily to me at all) is very helpful. If you have a lot of friends, it's very likely one of them will be able to help you market or sell in some way or another. I relied heavily on friends and friends of friends. I still do.

More specifically, figuring out who are major journalists or trend-influencers (potential customers who can convince their friends to become customers) within your target market and then befriending them is very useful.