What are the best practices for writing maintainable CSS?

Question

I am just starting to explore this area and wonder what are the best practices when it comes to production of clean, well-structured and maintainable CSSes.

There seems

Answer 1

I used an own order, convinient for me.

Rules there were listed in the descending order, and the criterion is rule's affect on the layout. For example:

.element {
    float:none;
    position:relative;
    top:-2px;
    z-index:100;
    width:100px;
    height:100px;
    margin:10px 0;
    padding:0;
    border:1px solid #000;
    background:#F00;
    font-size:10px;
    color:#CCC;
}

Of course, in the example above I didn't list all css instructions.

Particular idea was to keep order of groups, such as:

1. Positioning
2. Width and height
3. Margins and paddings
4. Borders (and others, affecting element's total dimensions)
5. Backgrounds, aligning (and others, which don't affect whole page's layout)
6. Typography