Eaton474
I didn't read all the comments, but, I will tell you why your wood is cupping, and how to stop it and/or fix it. I will be repeating what a friend and very well known and highly talented woodworker told me.

First, think of wood as a loaf of fresh bread. It can never be as dry on the inside as the outside. Even when kiln dried, the inside will still have a higher MC (moisture content), just like the outer crust of bread is drier than the moist, fluffy inside.
The one difference with wood, is it will always be this way to some degree. It may dry a bit more over time, but the inside will always have more MC, even 250 years from now.

Now, here is how to stop it from cupping after being resawn. There are several ways of dealing with this, each better suited to differing situations. You will need to learn over time which method to use and when, based on the MC content of the wood, it's stress level, how it acts while being sawn, etc.

First, you will need to allow all resawn wood to acclimate for at least 3-5 days after sawing, and a bit longer is better in certain cases.
Now, in general, if it's coming off the saw pretty straight and your just ripping it down the middle, just flip it back to back (the old outsides facing inward) and tightly tape the pieces together. Now, let them acclimate like this. They will pull against each other, and basically remain straight.
If there is a large difference in MC from inside to outside, lightly misting the original outside with water prior to taping them together will help equalize everything and allow it all to dry at a more even rate and keep the wood from pulling too hard. You can also mist the outside with water prior to resawing if it's cupping badlly due to the outside being so much drier than the inside. Using hot water, or even steam, will help the wood absorb the water faster and will work better in many cases.

If the wood cups coming off the saw, you can add a shim at the middle of the boards when taping them together to reverse bend them a bit. This will actually pull the cup out as they dry, and if you used a large enough shim, they will spring back straight after releasing the tape once acclimated. Again, you can mist the outside of the boards and combine the methods.

It may take some trial and error, but it's worth it to learn how to deal with the wood and keep it as straight as possible. Remember, it's easier to hold wood and keep it from cupping, than it is to reverse a cup once it's happened. And the straighter the wood after resawing and acclimation, the less you will need to machine away to get it back to perfect. Just remember, no matter how straight you get the wood to stay, it's almost always going to need some flattening via machines (ie: surfacing sander, jointer or planer) or with handtool methods.

There are other methods too. But the basic principal is to allow the moisture to equalize, and possibly help it along if needed. Wood will cup to the driest side of the board, as dry wood contracts and shrinks. Remember this, and you will soon have this figured out. Well, as much as is possible. Every piece of wood will act differently to some degree. It's just a matter of learning to deal with each situation in the best way for that unique situation. Understanding that wood cups to the dry side is the first step, after that, it will come to you in time and experience.