As you all know by now, I am by no means an expert on any of these subjects. But I am currently working on writing a sequel to my first book, Open Eyes, and have a few observations to share.
First, the first chapter is the hardest part. You have to give enough background to refresh reader's memories, but not retell the whole story of the first book. If it's been a while since they read it, you want them to still be able to follow, but you don't need to have a whole synopsis of your first book (another thing I'm working on, and it's driving me crazy).
Another tip for the first chapter: incorporate your summary into what's happening. For example, in Harry Potter and the Chamber of Secrets, JKR doesn't just talk about Ron and Hermione and how they're Harry's best friends. As Harry's trying to figure out a problem, he thinks of what Ron or Hermione would say, and JKR works in the summary that way. In my book, my main character, Kenna, is hiding from a man trying to kill her. As she's going about whatever she's doing at the moment, her mind drifts back to why she's in this predicament in the first place.
Sequels are hard, but they are doable. General rule I've found, jump into the story, and refresh the reader's memory when you think it's appropriate. But in my opinion, you should assume they've read your previous book(s), so don't give them a play by play of what's already happened.
What are your suggestions for sequels? What are some sequels that you thought were particularly effective? Ineffective? Are you working on a sequel?