Learn everything you can about what's now being called "growth hacking."
1. First, its all about the product. Create a minimum viable product (MVP) -- like a prototype -- and then present it to potential customers. Ask what they like, what they don't like, what they'd like to see, whether they'd pay for it, and how much they'd pay. You want to establish a perfect product-market fit so that it sells itself.
2. Integrate marketing into the product. Hotmail added a signup link into the footer of everyone's e-mail. Dropbox and other SaaS products offer referral bonuses to users.
3. Experiment with different marketing strategies to see what brings the most users at the lowest cost. Spend a little money on online ads, PR, and every other tactic to see what works the best. Once you see what delivers the best ROI, then pour all your money into that tactic.
Good luck! Let me know if you'd like to talk further.
PS -- I'm doing exactly this in my role as Director of Marcom at a high-tech startup.