In short, if you middle mouse button (MMB) drag from one frame to another in the timeline you can preserve the animation from that initial frame so that you can keyframe it on the latter frame. Let me show instead of just tell.
In this first image I have my character keyframed with an "f" sound at frame 29 that I am reasonably satisfied with.
I realize that I could use this same keyframe later at frame 65. Instead of trying to switch back and forth copying values or trying to manually reposition my characters face, I can simply MMB click, hold and drag the frame I want to replicate until my time line is on frame 65. Notice (below) that although my current frame has changed, everything in my scene that has animation on it is frozen as they were at frame 29.
Keep in mind that although I have his mouth controls selected, MMB dragging does not keyframe my changes automatically (even if autokey is on). I must do that manually by selecting the objects I want to keyframe and pressing the "s" key or by some other means. Notice the difference in the animation curves from the screen grabs above (before keyframing) and below (after keyframing).
Now that I have hit the "s" key the selected controls have been key framed at frame 65. If I now left mouse button drag around the timeline and return to frame 65 you will notice (in the image below) that the mouth is the same as frame 29 but all of my other animation remains as it was previously.
This is a quick and easy way to reuse animation from different frames. Keep in mind that you don't not need to MMB drag from a keyframe in order to replicate animation; MMB drag can used to replicate animation from any frame whether it is a keyframe or not. Best of luck.
