Very good ideas from Len.
Another way you can sketch, is the more painterly approach of drawing large regions of "roughly the right color in roughly the right shape". It comes off like a loosely formed lump of clay, and you then "sculpt" it into shape.
What I usually do with this is make rough versions of something I'm going to animate like this, and try to get the motion correct, before I do any of the detailing on the sprite.
When you're doing an animated character, the motion of the character is generally more important than the visual details on the character. If you waste time ironing out the details on each frame, in your first pass over the animation, you'll spend a ton of time working on it, and
to make any corrections to the motion, you'll have to redraw those details all over again, wasting tons of time. The idea is that you know in advance that you're going to have to go over and rework the animation several times, so you deliberately make the first versions "quick to draw" so that you can make this reworking process really fast.
For example, if it took you 30 minutes to completely draw and fine-detail a frame, and you were drawing a 6 frame animation, it would take you three hours to make just one pass at the animation. And it probably wouldn't look right, so you'd have to redo it a time or two, and pretty soon you've spent 6-9 hours making an animation that looks really awkward.
Instead, if you can cut it down to around 5 minutes to sketch each frame, you can sketch the whole thing in 30 minutes, and by the time it would take you to make just one pass over the whole thing (which is guaranteed to be "not as good as it could be"), you could have gone over the whole thing
six times. Once the motion's right, you're sailing with the wind at your back; you've already won. Literally, "the rest is just petty details".
