I'll tell all buyers to "find the seller's bottom line". Start low and be prepared for counter proposals. Its the best way to get a deal.
Let's put it another way - if a property is being listed at $100,000 and the most recent comparable properties have sold for $200,000, would you offer less than the $100K? I think not, as you wouldn't want to miss out on the property when someone else bid more than you.
Now, if a property is listed at $200K and comps show it to be worth $100K, I'm sure you're going to offer 50% off the asking price, as you don't want to overpay for the property.
Use that as a guideline - figure out what the property is worth, and then make an appropriate offer based on that.