More important than phone calls and visits is truthfulness.  Be open with your friends so they know the real you, even when you aren’t so proud of yourself. Friendships can only grow when honesty is present, so always be yourself. Always honor your friends.  If others are gossiping about them, either intervene or walk away.  Always keep their secrets. If you tell them that you’ll do something, do it without having to be reminded. For instance, if you hear someone talking badly about them, say “Hey, Monica is one of my good friends.  I’d appreciate it if you didn’t speak like that about her.” If you do something wrong to your friends, say you’re sorry to them.  Friendships are bound to run into the occasional conflict, and that’s okay. What’s not okay is allowing bitterness to fester and poison your relationship. Call them or pull them aside and say something like “I’m so sorry for making that comment about your mom.  I wasn’t thinking straight.  It won’t happen again.” Friendships thrive when everyone feels valued.  Let your friends know what they mean to you.  Tell them how thankful you are to have them in your life and compliment them regularly. Boost them and make them feel lucky to have you, too.  Take special moments like birthdays to be appreciative, but let them know you love them on regular days, too. Say something like “You’re such a great friend.  I don’t know what I’d do without you.  Thanks for always being there for me.” Just like you have your imperfections, know that your friends have them, too. Instead of trying to make them think like you about certain issues, accept their differences. This doesn’t mean letting them treat you badly, but do know that they are human and unique. For instance, your friend might be in an unhealthy relationship that you're tired of hearing about.  Instead of getting mad at them for staying, continue to love and support them.

Summary:
Be honest. Be trustworthy and reliable. Apologize when you mess up. Show appreciation for them. Accept them for who they are.