Okay, over the weekend I came up with an idea. It’s a pretty good idea, if I do say so myself, and it needs to be done. It’s patentable, in my opinion, and I’d like to see it get made.
This morning I did a little web patent search- and sure enough, it’s already patented- back in 1987. 
However, the patent’s run out- and it wasn’t renewed. 
So what’s that mean, though? Does this mean that anyone can make this thing, and get rich? 'Cause I’d sure like to be rich.
I think if the patent wasn’t renewed, it is considered to be in the public domain and you will not be able to patent it once more, as is. Having done some patent searches in the past however, I’ve been convinced that you can get another patent that improves on the original one simply by doing something as cheap as fastening a screw with a different screw driver…
Ok, Zor is 1/2 right. It is public domain. But the improvement depends on the original invention. Often in the realm of electronics and circuitry a minor improvement is enough to get a patent, but in the realm of most non-electronic devices the change has to be significant, you couldn’t patent say a left handed coffee mug or a 1/2 blue, 1/2 yellow sheet of paper (I know, they’re dumb ideas), it would have to be something like a self tuning guitar. There are exceptions, there was a mother/daughter team that came up with those icicle Christmas lights and they put a patent on it, but they didn’t explain the variations on it enough so companies were able to steal the idea by making modifications enough to not pay royalties (like making the “icicles” different lengths and such)
Patents, with some very rare exceptions granted by Congress, aren’t renewable in the U.S. You can’t go out and get a patent for the incandescent light bulb again.
You need to make an improvement that would be considered “not obvious to a regular practitioner in the art” involved in the invention.
Bob T’s right, insignificant improvements would fall under “prior art”, and be discounted. Depending on the nature of the invention, however, the innovator can patent the process by which it’s made, derivatives of the invention, or uses for the invention. Thus, the patent protection is extended far beyond the life of the original patent. It’s all about how clever lawyers for each side are.