Create a strong character. This doesn't mean your character has to be physically strong, but he or she needs to be interesting enough and likable enough to carry off several books. Your character will need a well-written backstory you can introduce to readers throughout the course of the book series. You want to introduce your readers to your character and get them invested in that character's life as well as the adventures you place him or her in.
Consider the title of your series. This will usually revolve around your main character. If you can think of a character name that will serve to title books in the series, so much the better. An example would be the character Stone, with book titles like "Stone-Cold Dead," "Get Stoned," and "Stone Sober."
Determine if the books in your series will have a running story that connects throughout the series or if you plan to make each book in the series a stand-alone story. If you plan to make your series one long story, you should start with a general series outline that breaks down the story by major plot points and have a good idea at what point in the series these major plot points will appear.
Keep changes to your main character minimal throughout the series. Readers of a book series typically stay with the series because they like your main character for who he or she is. Characters are always the main point of a book series and should never disappoint reader expectations.