Here, I have a few artistic tips for newbie tileset makers.

DISCLAIMER: This is a purely artistic tutorial. If you need any technical advice, go to Disguise's Tileset Resource instead.

CHAPTER 1: SHAPING

Lesson 1: A good alignment

A good tileset's tiles must be well aligned. This means every tile must fit with another, but there's more. When you create your tiles, don't excessively align their edges with the tile grid. We want to create a shapeful tileset, not an OCD orientated one.

As you can see, the second image looks more natural than the first.
Bad Example: MEZ01
Good Example: Medivo (JJ2)

Lesson 2: Taking shape

Be creative. Give your tiles a specific shape. If you can't imagine something, look at a picture. Go to Google Images and search for a picture, then try to replicate what you see. A shapeful tileset will be used more willingly by fellow users to create levels.

The 1st picture is nothing but a square with smoothed corners. The 2nd has a specific shape. Much better, isn't it?
Bad Example: Manufacturing Plant by me
Good Example: Space by Blade
In addition, try taking paper and pen, drawing a draft of the tiles you want to create, and then recreating them with your favorite drawing software.

Lesson 3: Going deep

Reusing the same tile and cropping the edges saves you a lot of time, but giving the edges a specific depth will make the tileset more realistic and pleasant to see.

Doesn't the 1st image look a bit too plate compared to the 2nd? Depth is important. In general, drawing something different on the edges will give a sensation of depth. You may want to include the unvariated tiles too, though, for a matter of versatility or to be used on sprite foreground layer.
Bad Example: Natures Ruins by Disguise
Good Example: Swamps by Agama

Lesson 4: Variations

Variate your tiles as much as possible, so it won't look like the world has been copypasted all around. This means you'll have to create more tiles of the same type, but with a difference in their final form. It will take time, but it's worth it.

The 1st one's tile is repeating seamlessly, but that's not what we want. We want it to look natural.
Bad Example: Evergreen Gardens by me
Good Example: Forest by Blade
Also, in the 1st tileset the soil on the sprite layer has been reused on the background layer, just with a darker color. To give your tileset more naturality, don't copy things from a layer to another. Or better, include both the copied ones and the different ones, so the player can choose which to use.
Bad Example: Castle
Good Example: Carrotus

CHAPTER 2: SHADING

Lesson 1: Border Lines

It is very important to trace the contour, possibly before coloring the tiles, because so players know exactly where are the floor, the walls and the ceiling. If you use a flawless shading, you might no longer need a contour in the end, but for now trace it.

As you can see, the 2nd image is more understandable than the 1st one.
Bad Example: River by Turtle
Good Example: Mountain by Disguise
Advanced Example: Fortress of Forgotten Sould by Blade

Lesson 2: Where's the sun?

Here things get a little harder. You'll need more than a color to make a shading. And even when you do, you'll need to make it look realistic. If the sun light comes from the upper right, use lighter colors on the upper and right sides of your tiles, and darker ones on the opposite side. You may even want to add a little light line on those to create a specularity effect. Also, don't smooth the shades more than necessary, or the shape will be compromised.

See? The 2nd image has a depth. The 1st one is a mess. Take a look at official tilesets to understand how shading works.
Bad Example: Ischa's Mine by me
Good Example: Chateau by Toxic Bunny

If anyone finds these informations useful, I'll write more.