For your thumbnails, which - if you're still newer to animation and are in your first five or six years of learning - should definitely be based off of solid video reference, I'd recommend using a VERY simple character (a stick-man is just fine - just make sure it shows the angle of the hips, the angle of the shoulders, the limbs, and the head direction) and drawing all of your key poses and breakdowns.
Study your reference carefully, and make a note and/or drawing of any moment that has a change in direction for the character or a weight shift or important gesture. Animation is so complex, and ignoring a footstep here or a weight-shift there can throw off all the body mechanics in your scene, so be careful to include anything important in your thumbnails!
I don't draw thumbnails for *every* breakdown, though. I won't draw a new thumbnail breakdown for a blink or a minor eye dart, for example, instead I'll just make a note of it on my planning sheets that I'll want a blink at frame 53 or some quick frantic eye darts at 102, or whatever. That's totally fine - the important thing is that you know exactly what you're going to animate before you sit down at the computer. As long as you feel confident that you are prepared, notes are just as good for that sort of thing.
For the major body moves, however, drawings will be much more helpful. And even if you aren't the best artist in the world, drawing him leaning over his left foot is going to be a lot faster than describing in notes all the different things that happen in the body when someone leans over their left foot.
Last, I want to recommend a great book for this stuff called Simplified Drawing for Animation Planning by Wayne Gilbert. Wayne's book is invaluable for this stuff, and actually covers a LOT more than just thumbnailing - check it out at: http://anamie.com/anamie_buy_now.html
Hope that helps! Thanks for coming by, Angga!