Speaking as someone in a sole relationship with someone who has a primary partner, here would be my $0.02. Hope it helps.
- Assume good intentions - Most secondaries get into this with their eyes wide open. Share anxieties and worries from both sides. All parties can probably all make simple changes to improve the dynamic when needed.
- Keep in touch - Not every detail of primary life, but nothing feels better than getting an impromptu "Thinking about you. See you soon." message.
- Encourage Friendship among partners - Putting a human face on someone makes a big difference. Being friends (although not a requirement) with your partner's partner really rallies everyone in the right direction since the combined happiness of the group becomes critical - driving all the right behaviors.
- Plan, plan, plan - As a secondary, I still have a hard time with sometimes feeling like I'm getting the primary's leftovers. Simple things can avoid this heartache, like scheduing and being up front about priorities.
- Don't let things slide. Address issues immediately. Most likely, they'll re-surface.
- Don't be an ass. Sounds simple, but recognize the complexity of your relationships and the additional reassurances and gestures that need to come with it.
- Don't lie. End of story.
- Don't make it any more complicated than it needs to be. Often, it's easy to get sucked into problem solving all of the time, when really focusing on having a good time and living it will make things feel better for everyone.