Friendships that last have common traits, a few of which include:

You have a few close friends.
Some people have a large network of acquaintances, but very few close friends. The friendships that tend to last are the few close friendships you cultivate. Instead of trying to collect a large number of superficial acquaintances, work on developing a few close friendships, and then cultivate those friendships. This type of friendship tends to last much longer than acquaintances that come and go.

You're there for each other.
Friendships that last are those that know they can depend on each other. If you are always there for your friend in a time of need, and he or she is always there for you, it builds a relationship of trust and caring, essential for a good, lasting friendship. If you want your friendships to last, be there for your friends.

You work at it.
Whether it's calling a friend to see how her first day at work is going or taking a friend out to lunch as a thank you for babysitting, friendships last when you put effort into them. Make sure you are a friend to your friends and go out of your way to make them happy and know you care about them and appreciate them, and your friendships will last.

You don't let distance get in the way.
Many times, friendships will dissipate and you will grow apart after college, after you get married, or when you move. However, friendships that last don't let this get in the way. With the advent of social networking sites like Facebook, email, and unlimited weekend cell phone plans, there's no excuse to not stay in touch with friends. Even as you go your separate ways, make it a point to connect with each other on a regular basis, whether it's a phone call or a lunch date.

You share things.
Close friendships develop and last when you share things, whether it's going to a friend for advice about something personal or sharing a fun experience together, like a trip. Opening up and confiding to your friends helps strengthen friendships and increases the likelihood that they will last.

You make your friends a priority.
True friendships that last are a priority. Make your friends a priority and be there for them, whether it's returning a phone call in a timely manner or giving up a Saturday morning to cheer them on at a race they are running.

Know when to cut them loose.
If you are working at being a good friend and not getting a similar effort in return, it might be time to find a new friend or work on developing friendships that are reciprocal in nature. Don't waste your time being a good friend to someone who uses or doesn't appreciate you.

If you are doing these things, you know that your friendships will last, no matter where you are or what you're dealing with.