In my experience, you will ever have tension due to the difference in cooling from the bottom, to the higher part.
If you dont want to use a heated bed, to even out the cooling, the only option is to force the part to stay in shape by spreading out the surface the part sticks on. This will not reduce the tension, but it will reduce warp significantly.
If you mill cetrain plastics, as acrylic for example, that hasn't tempered properly, then this tension warps the part like hell, when you take off a few millimeters from the surface.
As Daid said, a raft would help... personally I dislike the rafts, as it takes ages to print them and they are a real waste. That's why I prefer the discs in the corners.