Never belittle someone who has a passion for tropical fish, stamps, trainspotting or the history of pipe organs in Eastern Europe (***see "indulge your love of old movies, crochet, darts, regardless of whether they are "in". ")
If your friend is undertaking a project that is near and dear to them, then be interested, even if it is about pipe organs in Eastern Europe.
If a toe nail turns a funny colour, fix it before you end up with 9 toenails.
Know you will feel a little envious when your friends lose weight, get a fabulous job, meet the perfect partner. This is ok, just don't wallow in it or show them.
Know that you will feel smug when your friends gain weight, lose a fabulous job, or get dumped by their fabulous partner. This is ok, just don't wallow in it or show them.
Learn to say "Congratulations!", "I'm sorry" and "How can I help?" and mean it.