To your partner's family- You need to treat them like you own with the same respect and gratitude that you give your own parents. The easier you are in their company, the more relaxed your partner will feel. It all filters down to them so welcome them with open arms and your partner will love more for it.
To their family
To your SO's friends- Again, treat them as you would your circle of friends- it makes social gatherings better for everyone involved because your partner can mingle without feeling that they have to babysit you all night and vice versa.
New ideas- Always welcome new ideas- you may not agree with them or see them as a good idea at first, however the most important process for you as a couple is how you discuss it and talk about the possible outcomes and consequences of pursuing this new idea.
Change- Couples have to go through a lot of change whether they want to or not. Always seeing change as a negative can put a massive strain on your relationship. Try to find the good in every change you face- whether that's a new job, a house move or a shift in your relationship. Positives can arise from the most uncertain of times.
Gestures- If your partner has done something for you that isn't quite the way you would have done it- welcome the gesture anyway. They thought they were doing you a favour by completing the task so say 'thanks' and diplomatically tell them how they can improve next time.
When they arrive home- A cheery 'welcome home' shows that your partner isn't coming into a place of hostility but one that's easy and relaxed. Mostly it shows that you are happy they have come home to you.
When they thank you for a gift- If you give them something on a birthday or at Christmas and they thank you for it- say 'you're welcome' as a means of acknowledging their gratitude. It shows them that you like to spoil them every now and then because you love to see their reaction.