I am super picky and not always seeing someone, but when I am, I like to see them once a week. Making the other person feel valued and primary is extremely important to Ben and me. We spend most of our free time together and try to be intentional about the quality of that time (e.g. We've both acknowledged that if our foundation wasn't strong, we would probably feel more jealousy. The biggest rule is communication - we try hard to balance respect for our other partners' privacy with open communication between the two of us. Everyone knows right off the bat that we are happily married, and thus not looking for a lifelong commitment.
In the past I have spent the night at a boyfriend's house, but Ben has not spent the night anywhere. We shared a big laugh when we realized just how weird/unusual it is for a husband to comfort his wife about her breakup with a boyfriend. We also feel strongly about treating the people we date with respect and care (and expect to be treated the same).
We took it really slow (lots and lots of conversation) and occasionally dated outside of our relationship.
As time went on, the "casual" piece has become less and less important.
We talked about nonmonogamy in theory for a long time (two years? In hindsight, I feel like this gave me time to get used to the idea and for us to build a solid foundation.