How to get a solid comp?

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When I did my first deal I got my comps from online websites such as Trulia. Of course, that did not go so well and the price turned out to be very high. Now I have a realtor who pulls MLS comps on a wide range of properties with rules like don't include properties crossing major highways, more than 5 miles away, has 'handyman special in the description'. My comps are getting better but they still seem too high on dilapidated properties. What should I do to get more precise comps? Could I add more MLS comp rules for my realtor to apply?