Texturing Principles

Simplify - Break down the details into simple abstract shapes. Don't try to depict the fine details of every leaf and blade of grass. If you have the resolution to do so, you're probably not making pixel art.

Repetition - A good texture may only need a few simple clusters. Repeat these same clusters over and over but vary the pattern in which they are distributed.

Balance - The texture should feel balanced. A little detail here usually means you need a little detail there. It's all a balancing act.

Contrast - It looks more interesting to vary a texture, rather than have the same consistency across a large area. This can be achieved by placing texture details in select areas and leaving empty areas where the texture is implied. Or, you can achieve textural contrast with color variation. A perfectly homogenous texture can appear unnatural and too busy.

Avoid orphan pixels - Orphan pixels are single pixel units not connected to a cluster of the same color. They can distract the eye and are usually avoided. However, I permit them when they're not obviously floating alone and relate to a greater texture, in which case I don't consider them true orphans.

Here is a simple tutorial on making a grass texture, which illustrates all of these principles.