Buying/selling real estate: Earnest Money

I am always in the market for real estate. Usually buying but I have sold a house too. When I sold my last house, I didn't require the buyer to put any 'earnest money' down on the agreement. After reading this blog, I think I made a big mistake that could have cost me dearly. I should have required earnest money so that the buyer would be more likely to stick to our agreement.