Aqusition phase of investing

When purchasing a property, what kind of Cash-on-cash return do you guys look for? What numbers make you say, "yes, I'm buying this property," and what numbers make you say, "no, this deal isn't for me." When I analyze properties, all I do is figure insurance costs, prop taxes, and the mortgage and 5% vacancy and annualize it and subtract that from whatever my annual total rent is.. Is there anything else I'm missing when "running the numbers"? I don't know too much about what returns you should be looking for; I look at the bottom line, "am I cash flowing or not?"