Always keep your CSS clean and organized. I like to organize my selectors into style categories.
reset styles
Typography styles
layout styles (header, content, footer, etc.)
module or widget styles module or widget styles
Others (etc).
Then, within each group, I organized the selectors according to DOM hierarchy:
any parent styles
block-level element styles (paragraph, list, etc.)
Inline element styles (links, abbreviations, etc.)
Others (etc).
Then make specific divisions based on the above divisions:
paragraphs
blockquotes
addresses
Lists
forms
tables
Others (etc).
Finally, I organize the CSS declarations by style type
positioning styles
Float/clear styles
display/visibility styles
spacing styles (margin, padding, border)
dimensions styles
Typography-related styles (line-height, color, etc.)
Miscellaneous styles (list-style, cursors, etc.)
Many people like to sort alphabetically by declaration, which may not work well for me, but may work perfectly for you. But once you choose any organizational method, it's best to stick with it to maintain its uniformity.