Variety

I found that it is very interesting to set oneself a “theme” from time to time and consciously stick to it (of course without missing out on other great opportunities...).

Examples of such themes might be: “reflections”, “animals”, “shadows”, “houses”, “doors”, etc.

This helps to improve ones technical skills (esp. if you discuss your photos with others) and also, you won’t overlook similar subjects in the future.

Also, you could try using different formats (“landscape”, “portrait”), include wide panoramas, but also special details, try a photo with and without persons in it, change your position (photos of children and small animals are usually much more interesting when you get down to heir eye-level, for example) and extreme perspectives.
Using various focal lengths from the same position (eg. a strong tele lens or an extreme wide-angle) may also result in very different images.

After all, isn’t variety the “spice of life”?

Shameless plug: You find some of my thematic galleries at Online Photo Galleries

How to take good travel photos