The greatest resource I have had for advertising is a dentaltown course called Marketing Summit Series. ($160)
They outline where to pay for advertising based upon ROI. The way it works is tiered: start with internal referrals, then website and pay-per click, then direct mail, and only after those should you think of radio, TV, or digital billboard as those are the highest risk with possibly the lowest ROI. Obviously any marketing is dependent on how marketable YOU are, so you'll need to find a niche to market like Fear (sedation), implants, kids, price, etc. If you advertise that you're just like every other dentist in the area, it won't help regardless of how much money you spend.
Hope this helps! Let me know if it does!