If there's one CiviCRM feature that pays back the time spent learning it, it's smart groups. A normal group is a fixed list you add people to by hand. A smart group is a saved search — “everyone who donated in the last 12 months but hasn't renewed their membership”, say — that updates itself every time you open it.

Once you've built a few, mailing the right people gets a lot less painful. No more exporting spreadsheets or double-checking who's who; the group just reflects reality.

Start small: pick one segment you re-create manually every month, and turn it into a smart group instead. Once you've done that once, the rest tend to follow.