Never design a tileset in a vacuum, always have a scene/level/whatever in mind. What is this tileset for? If you don't have a use in mind for it, then why are you making it?

Try starting not with your tiles, but with a mock-up. Draw a sketch of one of the scenes you plan to eventually make with this tileset. Keep it to scale, but don't worry about it tiling well at this point. Draw it in as much detail as you feel up to - what will the edges of the map look like? Will objects cast shadows? How will buildings, trees, etc interact with the ground, if at all? Draw it as you would an illustration, don't skip something just because it might be hard to do in tiles - forget about tiles for now.

Once you have that mock-up, you can look for patterns. What are elements you can repeat? What are things you can fit nicely along tile boundaries? Move things to fit your tile grid a little better. If it's a side-view tileset, how few different slopes can you get away with?
With a tile grid overlaid on your sketch, it gets a lot easier to see what tiles you need.

Then, you can start making the actual tiles on top of the mockup or separately from it, and rebuilding that mockup with your tiles, piece by piece. Working with a mockup like this also makes it easy to check as you work that your tiles actually convey what you want them to convey.