To Sequel, or Not to Sequel?

Recently, I've been contemplating series and sequels. My DH says, "Write a series -- that's where the money is." (As if I am writing for the money... do I wish I was J.K. Rowling? Absolutely, lol... realistically, however, I write because I have to).

I've frequently read books that I wish had a sequel or two. I'm also a fan of one or two series (the "In Death" series by J.D. Robb being my fave), but I've also seen series that run out of steam, and I've stopped reading (the alphabet mysteries by Sue Grafton being one, for me at least... I stopped at "O").

So, do you plan a series? Or not? Do you plan a sequel? Or not?

I know, I know ... baby steps, and one thing at a time and all that, but I think writing a book that is intended to be the first in a series requires different planning than one that is not.

On the other hand, planning a series seems a bit presumptuous for a novice author... though that didn't stop J.K.!

What do you think? Series? Sequels? Or not?