@stephthegeek and I are polyamorous and cultivate multiple relationships. It can work really well with two major tools:
Intentionally over-communicate. Talk about everything. Share all of your feelings. It's ok to be conflicted. Every feeling of jealousy is an opportunity to work towards compersion and become a more generous partner.
Google Calendar. Calendar everything. I can't stress enough how likely you are to double-book dates.
I would not have thought of #2. I suspect that may be a great help.