It may sound like funny advice, but I thought JK Rowling was really good at doing that, so go read your HP books :-P Remember how she'd mention a character, then describe, in one little sentence, something they had done in the past that was relevant to the current situation? It's short and it's relevant, so it's not obtrusive or annoying for readers who are familiar with the story, and it's enough for the reader who's been a while between books or the reader who is new to the series.