Word of mouth
Don't get me wrong you will have to do traditional advertising as well to supplement word of mouth, but the best marketing tool, is still word of mouth. It's cheap and powerful; isn't that what you want from marketing? (Especially as a cash strapped startup!)
Despite this, getting positive word of mouth is often overlooked as a by product. A magical occurrence akin to going viral, that just happens if you're lucky, because we all feel that people should be lauding our product to everyone they meet right?!
But word of mouth occurs when a customer's expectations are greatly exceeded.
Offer something so good that people will share.
It's also probably the hardest to master but if you do a few things you'll do pretty good at it.
Product - does one thing exceptionally well and is simple for the target user.
Service - customer service needs to exceed the minimum that you come to expect. It's not only being there if their is a problem but going above and beyond. It's not only dealing with problems, exceeding expectations when generally when dealing with customers will help too. Remember, sometimes it's the little touches which go a long way.
Memorable - Being easy to recall in a relevant situation is massively important! No use having someone sell the product as amazing and then not remember the name or the brand. Part of this is the experience and impression it has left on someone but they then have to remember the name in the moment too.
Oh, and don't confuse word of mouth with social media, the clue is in the name, 'word of mouth!'
So next time you think about how many people are on your marketing team, remember to add the number of customers you have.