For me the biggest goal is fewer lines to organize in the first place rather than any particular scheme for grouping. What I do is:

1) Use as few separate stylesheets site-wide as possible so any contradicting or repetitive rules will be obvious.

2) Use one line per rule so I can scan the selectors quickly, then scroll horizontally if necessary

3) I prefer complex comma-separated selectors which set one or two properties to simple selectors which set many properties. Grouping in this way gets me closer to having constants, e.g. a specific hex color won't be rewritten again and again, it will just follow a complex selector list.

4) Once a selector works, I try to make it more general, e.g. "div.info { font-size: 12px }" can probably just be ".info { font-size: 12px }". More general rules will apply in more cases, so fewer overall rules will be necessary.