Tips: Writing a Series

Knock one over and…
I’m working on a new episode of One Fictitious Moment (check out the first four episodes here) and the topic I’m researching is how to write a series.
Here are some of the tricks I keep in mind when writing a story that is part of a series:
Always remember there is a larger story-arc and try to keep pushing it along at regular intervals.
You have to give people a reason to read the next book. It doesn’t always have to be a cliff-hanger, but there has to be a reason to come back.
You don’t have to retell the stories prior to the one your writing right now, but you may need your characters to ‘remember’ certain events if they are significant to the current story. A flashback or a quick memory written on the page can work.
Remembering all your little details is really important as is all the foreshadowing you need to drop in so that by book 4 your reader smacks their head and goes “Ohhhhh!” – thanks for these two tips Amy from Inkcouragement!
What are some of your best tricks for writing a series?