Here is how you do it.
First, I have a cube in "animation" mode. I inserted a keyframe at frame 1 (I Key->Rot).
I then moved to frame 200 and rotated the cube 180 degrees. Insert another rot keyframe. Since we are only working with the ZRot key it is safe to delete the XRot and YRot keys by selecting them in the IPO window (the colored buttons to the left of their description).
You can edit and move curves in the IPO window just like you can in the 3D window. I maximized the IPO window to show you. TAB itno edit mode for the IPO window and you will see the ZRot curve turn yellow providing you with handles to move the curve just the same as a bezier curve would move.
As a side note, when we rotated the cube in the positive direction, the curve went negative. This is because the curve represents rotation clockwise / counterclockwise. A positive rotation in the 3D window is clockwise movement hence negative curve.
Now all that you do is G KEY grab the bottom handle making sure you have the CENTER dot clicked. Constrain on the Y-Axis (Y KEY) and type in -360 to go exactly to -360 degrees.
We now have a cube that spins 360 degrees exactly once over a 200 frame animation. There are two problems with it though. First is that the cube will start off slowly and spin faster as we get closer to frame 100 then begin to slow down again as we approach frame 200. This is because of the curve type. To make the cube spin at a constant speed, we need to change the curve to be linear. Under the curve menu, go to "Interpolation Mode" and select "Linear". Your curve should get less curvy.
So this takes care of one rotation but what about if you wanted it to spin multiple times? The answer is to repeat the steps you did above which is time consuming. Blender has simplified that process. TAB back out of edit mode for the IPO window and go back to the curve menu. Make sure the ZRot curve is selected first. In the curve menu is "Extend Mode". If you choose "cyclic" it will repeat your curve over the entire life cycle of your animation no matter how many frames you have.
Since the curve now breaks up, it doesn't spin backwards on itself.