One "secret" to underdog marketing is to harness the power of continuity.
Put simply: don't change up your logo, messaging, colors, ad campaigns, et. al. very often. People need time to get comfortable with your look and story...with you. How many songs have you disliked like the first time you heard them on the radio only to buy them on iTunes a few weeks/months/years later? I have a list of them.
Your advertising only really starts paying off when people can register your message without having to read it. They just get it instantly, viscerally from the sight of your logo, icon or colors. But that can only happen if you constantly and consistently reinforce the same "own-able idea." This, of course, requires that you think through your positioning before you begin executing.
Yes, you will tweak your message and your look as you learn more. But 85% should be working from the get-go.