Honesty is key. In a successful friends with benefits situation, both parties need to be upfront about their expectations. For example, if one person is only interested in a short - term physical relationship and the other wants more emotional connection, it won't work. Another element is respect for boundaries. They should clearly define what is acceptable and not in the relationship.
One major element is trust. They need to trust that the other person will keep their word about the boundaries and not try to change the nature of the relationship without prior discussion. Also, a shared sense of fun and enjoyment in each other's company is necessary. If they don't have a good time together outside of the physical aspect, it's less likely to be a success. Additionally, being able to separate the physical relationship from their friendship is important. They should still be able to be good friends without the physical part if the arrangement ends.