When Writing A Series...

1. Decide even before you begin the first book whether it will be a part of a series or not.
2. Treat each story as an individual book, focusing only on one pair of characters. 
3. Stir readers' curiosity about your other characters (the lead/s on the next story/ies) by making them 'look' and 'sound' interesting. Introduce them but don't give too much details, to the point that they outshine the leads.
4. Make sure that majority of the scenes in a novel will focus on the lead characters of that particular title. If a minor character will appear on more scenes than the major leads, it might confuse your readers.
5. Use the point of view of either the hero or the heroine.  
6. Each story must be different from the others. Different characters, different personalities, different goals, different conflicts; different gestures, manner of speaking and ways of thinking, too.
7. Do not overwhelm the readers with too much details.
8. Keep a series 'bible' to make sure there are no loopholes or inconsistencies, and timelines match. 
9. There should be a central theme to give your series its own sort of 'character' or identity. *
10. Give 'parallel' titles. *

*Note: For marketing and aesthetic purposes. :-)