The grass tilemaps that I use for grass looks very repetitive. Basically, I am taking a single repeatable grass texture and repeating it over and over again.

I really like playing Heroes of Might and Magic 2. It's an old classic with 8 bit graphics. The thing is, their terrain tiles looks great. It's very hard to notice the repetitions. Attached are two blank maps I created with their map editor:

Create a number of different variations that tile together, and select randomly.

Yup, and also make sure your terrain tiles aren't too "interesting". The stronger the shapes/details in them, the more the pattern will be visible.

The tiling will also become less noticeable if you can add random things on top of it, like rocks, bushes, etc....

Getting more complex. you could blend two tile layers together, or vary the lighting/color slightly of each tile.

Basically, making tiles not look repeated is a bit of an art.

One simple trick used in 3d terrains is to sample the same texture twice with different uv scales and blend (modulate for example) the samples together. This will reduce the tiling effect considerably.

Yes, that is a very good approach. Simply use the uv coordinates of your geometry and use perlin noise or other to blend grass, rock, dirt, etc.. together. That with proper mipmapping will go a long way in making your terrain interesting to look at instead of an endless world of the same texture tiled over and over again.

Create a number of different variations that tile together, and select randomly. If you wanted to get more complex, you could use a Wang tiling scheme ( http://en.wikipedia.org/wiki/Wang_tile ) but doing so would complicate both your tile generation and your auto-tiling filler routine.

On the other hand, placing variation features (e.g. flowers) across tile edges and corners and not only in the middle of tiles requires having more than one edge type. How far do you need to go to break the grid depends on graphical style.