Occasionally as professional animators we may be called on to fix other animators' work. This is not uncommon during the crunch of production. It is always with some trepidation that I look under the hood of another animator's shot; it's kind of like going into someone else's bathroom. Most of the time it's okay, but once in a while you come across somebody who obviously hasn't bothered to clean up in there in a while, and now I've got to tip-toe around and try to work with this mess. There could be tangent handles shooting off in all directions, keys that were left in held or linear mode, and jagged splines that necessitated counter-animation in other areas to make the animation look smooth. I'm pretty anal-retentive when it comes to splines, so this kind of thing really makes my jaw clench. I'd like to take some time here to go over some basic concepts of spline hygiene, just in case I ever have to fix one of your shots. Practicing basic hygiene can prevent a lot of problems with timing, spacing and arcs, and can give you a head start towards your polish phase. It's also just good manners!
Here are some typical dirty spline problems, and how to clean them up (Roll-over the images below to see "before" and "after"):
Smoothness - Are the tangents rotated so that the splines pass smoothly through the keys? Do the keys allow the splines to describe natural, pleasing shapes? Just like the arcs and paths of action in your scene, the splines should have a pleasing fluidity to them.
I will even smooth the tangents when my keys are on one's. It makes the spline easier to look at, and if I need to stretch out the animation later on, the new in-betweens will likely do "the right thing".
Curve containment - Do the splines pass beyond the values of neighboring keys? You should let the keys to describe your extremes, not your curves. Otherwise your extremes may change or disappear once you start offsetting keys in time. It's also easier to keep your animation organized in a dope sheet editor when you know that the extremes are represented by keys.
You should also try to keep the spline handles from overshooting other keys in time, as this will give you hitches in the motion.
Economy of keys - Do you have a lot of extraneous keys? Sometimes it's better to delete some of the extra keys on a curve to let the computer smooth it, and therefore give you smoother motion. Once your blocking is approved, it's also a good idea to clean out all the redundant keys (those that have flat tangents and are the same value). This will keep your splines looking cleaner and easier to edit.
Variation - Are there repetitive curves in your splines? If something looks repetitive in your splines, there's a good chance it looks repetitive in your animation as well. Try to add variation to the value, tangents and timing of your splines to break up the animation subtly.