I haven't done spriting in a long time, but I'd agree with using a graphics tablet. Basically, you can just draw out all your sprites at a really high resolution, rough the colors* (you'll need to decide on your palette beforehand**), shrink to the proper resolution***, then fine-tune and add details (again, paying attention to the palette you decided on). Personally, I've always liked
Monoprice Graphics Tablets for both their price and feature sets, though I grant that the pen could be made a bit better. All of their graphics tablets should be well below 150 BGN (closer to 75 BGN), and I've been quite satisfied with the model I own. I'd recommend at least giving it a shot, if you're interested.
*At this stage you're interested in just getting the lighting right, so that the basic forms of your sprites show through. Don't do highlights and other details at this step because it'd all end up wasted after shrinking (for the most part).
**You can change the palette as you continue to work with the sprite, but the idea is to remember to work in discrete colors, rather than continuous ones. A good example of this not being done as well as it could have been would be
Finn's sprites in the DS Adventure Time game (very evident in his ducking sprites).
***Depending on how high-resolution you're talking and the style you're going for, palette concerns may not apply at all. If you're going for a Super Street Fighter II Turbo HD Remix look then you probably don't need to pay attention to your palette or do any fine-tuning beyond what you'd do for just regular art (though you should always work at a higher resolution than the intended display resolution, in my opinion). On the other hand, you could still do HD but go with a style similar to Blazblue (which is just a tad smaller resolution than SF 2 HD Remix) or KoF XII/XIII, in which case paying attention to your palette and fine-tuning would certainly be musts.