Well, I guess I should have one of these sprite topic things.
I'm not exactly the best at bases, but I'll give 'em a go from time to time. I will help me practice, and get better.
Here's my first, kind of okay sprite base for the year, Luke Skywalker.

Duck Hunt

• Duck Hunt Dog

Earthbound

• Ness

Family Guy

• Glen Quagmire

Powerpuff Girls

• 1st Him
• 2nd Him
• Ramsey's Him

Star Wars

• Darth Vader
• Luke Skywalker

The World Ends with You

• Neku

Three immediate problems, light sourcing on legs is border line pillow shading and the wrong foot is forward, the anatomy of the face is pretty rough, the readability of the arms and hands is unbearable. Other than that you seem to have a pretty good grasp of basic spriting and shapes.
- Try to use a light source, perhaps draw a circle that represents where the light is coming from to help
- Look at other sprites, or even images of people in order to get the anatomy of the face correct
- Try to sprite the arms separately, starting by giving him more than one shoulder (In fact if you have a program with layers you might get away with thinning up and sprucing up the current arm, then copying it to the back and touching it up to line up.)
If you want me to poke around at it to give a more straight forward example just ask :D

Yes please!
As a dev, I need to be able to make bases better, I can animate and do stuff once I have something to work with, but I would like to be able to make my own custom things better.
I've gotten lazy, I need to retrain.

Lets start with the face.
First of all, our skulls are generally round with a square jaw. You're rocking the oval.
A little bit of widening can fix that straight up.
But wait a second... where the heck is his neck? Where the heck is his ear? WHAT DO I DO WITH THIS EMPTY SPACE ON THE SIDE OF HIS FACE??
Don't be afraid to get rid of outlines if their not on the outside of the sprite (hence outline), if you have every part of your sprite outlined you're going to start losing space for detail. A few pixels later and he's starting to look more human.

Sorry if I'm babying you but I'm having fun being dramatic, next up, legs!

Oh wow! You've really made that better already. Keep the tips coming!

Next we work on some anatomy for the legs.
First of all, if you're stand to the right(as a sprite), unless you're legs are on backwards, the right leg should be closer to the screen(if that makes sense, think of Luke's perspective). This creates depth and perspective, if your left leg is closer forward it looks as if they're trying to walk across a tight rope. Secondly, the waist of a human being doesn't start at the bottom of their clothing, not when they're wearing a robe at least. So you pump those legs up to where the waist is supposed to be. I assume this happened because you drew the shirt and then the legs later, you need to draw in layers unless you know exactly what you want, starting from a naked body(maybe not with too much detail) and up.

I won't be messing with the shading or details yet, but man that belt has some problems xD

I really do need to get myself a layering program. Fireworks sounds nice.

I'll test out what you tell me by doing Darth Vader next.

Sounds good, I highly suggest Paint.net, I can send you my default config for spriting.
It's just a pencil tool with the secondary colour transparent, so it's an effective eraser. Than whenever you choose a colour with the colour dropper it switches back to pencil, so just have the pallet on screen. It's basically the perfect setup. The arms getting fixed will take a bit still because in order to show how I'm fixing it is quite ridiculous xD

This didn't turn out perfectly, mainly because the pose was pretty rough to begin with. I would change the pose to a one handed style with the light saber behind his back, and his hand forward. However, I showed the concepts pretty nicely so I'm going to share anyways :)

As for shading and textures, it could take a whole year in order to teach you, simply because their are so many different techniques and ways to shade and texture. I suggest looking at other art or tutorials for help there :) I hope I helped with your future anatomy drawing!

The way the lightsaber is shaded makes it look like a plastic stick. I recommend looking at some sprites from the Star Wars games to get an idea of how a lightsaber could look in pixel form.

And here I kick in to go and help with shading.

As I can see, you have a base concept of light source, but you have a problem with texture. Can I use that base you did Nick and play with the shading?

Of course :)

Okay, all advise for Luke has been noted, I'll have a go at fixing the belt now too. Any suggestions before I do?

But here's a little something else I had a go at.
Glen Quagmire.

The arms look a bit strange to me... also the sprite seems to be missing shading. Either that or there is so little shading that it's barely noticeable.

thought you said Vader next.

Lol Quagmire looks weirder than on the show.