Compersion, and erotic compersion, are great, and are emotions I've had right from my first poly relationship (in which I was a metamour). I think great deal of it had to do with the fact that I was in an amazingly stable relationship, with very frank and clear communication, and there was a strong sense of trust and security within that relationship. Like Becca, I have also been fortunate in the relationships I've been involved in, in that I did not feel threatened in any way by my metamours, who made it clear that they had no intention of undermining my relationship with our mutual partner. At the end of one relationship, when things were rather thorny between my partner and his ex-partner, I felt more exasperated and annoyed than compersive. However, that was nothing to do with jealousy over sexual or emotional contact, but because the relationship was turning into a burden for all concerned.
I also think I was less understanding than I should have been over the difficulties my metamour was experiencing. This was her first poly relationship, and her first relationship that involved such deep communication, and I felt impatient over her lack of openness and over her jealousy. Looking back, I should definitely have realised what a steep learning curve she was on, and been more compassionate. While I was never snappy with her, I feel that my impatience affected my own emotional state negatively, particularly towards the end, and needlessly so.
I firmly believe that a sense of total security with your partners is a necessary pre-requisite for experiencing compersion, and that having a total sense of security with your metamours runs that a close second. Anything your partners and metamours can do to help you feel cared for, valued and necessary in their lives will increase the liklihood of experiencing compersion. Regularly feeling taken for granted, overlooked or 'in their way' will increase the liklihood of experiencing jealousy. Having said that, we need to bear in mind as standard that our partners and metamours are not mind-readers, and we may need to tell them what we need to help us feel more secure (we need to take their needs into consideration as well, and not throw a wobbly if they can't give us exactly what we feel we need, but recognise and appreciate when they are honestly doing their best).
Another necessity is setting aside the cultural conditioning that insists that each of us can only love one person with all of their heart. This can be difficult to overcome, but is necessary to establish the sense of personal security within our relationships that I believe is a pre-requisite for compersion.
I think it is unrealistic to expect compersion from someone in a new primary relationship, unless they are already in another long-term, happy, stable, primary relationship. That person's emotional security is entirely tied up in an intense new relationship, with no safety net should they fall. The fact is that the early stages of any relationship are bound to be less stable and more fraught with difficulties than the later stages, when you have adjusted to one another - any one of us only has to look back at the first couple of years of a primary relationship to recognise that - and when that is all you have, it is an emotionally unstable place, with no emotional support except from that same relationship. This is not necessarily the case with a new secondary or FWB relationship, as the emotional stakes are less high.