She calls every Friday and Saturday night and talks for hours about her problems, not allowing you to get a word in edgewise. Apparently it is completely lost on her that you are also going through a difficult time and that you, too, need to talk. Your heart goes out to her that she is suffering, but she is honestly driving you crazy with her clingy behavior. How do you set boundaries without hurting her?
Healthy relationships are a balance of give and take. Both friends have a respect for each other’s boundaries and understand that even the closest friends need time apart. Maybe your good friend is going through a difficult time and needs a shoulder to cry on. Sure they are a little needy right now but you give them your support because you know when the time comes they will do the same for you. Neither one of you are keeping score, anyway. 
Sometimes, though, you develop a friendship with someone who becomes very demanding of your time and energy. At first they may seem like a well-balanced person, but gradually they become increasingly clingy. Soon you find that your friend is draining energy from you with their constant demands for attention. Dropping by unannounced, inviting themselves along on outings, constant texts and lengthy phone calls are becoming the norm. In a healthy relationship it is possible to be supportive without sacrificing yourself, but with a clingy person you may find that you are giving away more of yourself than you would like. 
There are plenty of reasons why a friend would become clingy. Perhaps their best friend has just moved away, they are new in town, are going through a divorce or just have had death in the family. Do not assume that they are being disrespectful of you if they are going through some real difficulties. Treat your friend kindly because if they have recently been through a trying time, their clingy behavior may be temporary. However if they have not experienced trials lately then there may be deeper reasons why they have become clingy. For example, they may have had a troubled life and have developed an anxious style of personal attachment, needing constant reassurance that they will not be abandoned. 
If you are beginning to feel that your friend is taking advantage of you and you find yourself feeling resentful, take this as a sign that it is time to set boundaries with them. This may be difficult and take some courage on your part. No one wants to be the bad guy and cut off a friend in need, but to preserve your own health and even possibly the friendship itself, limiting the other person may become necessary. It is okay to give yourself permission to take care of yourself and not give into feelings of guilt or the fear that the other person will get angry. In addition, setting boundaries is sign of self-respect. If you respect yourself, others tend to respect you more and maybe your clingy friend will, too.
So what can you do to give yourself some space?
Instead of being overtly direct with the friend right away, try discretely creating some distance between the two of you first. Take a day or two to respond to their phone calls or texts. In this way you are setting a clear message that you have a life apart from them. If you respond to their demands right away, you may be sending the wrong signal that you are available to participate in their needy behavior.
Introduce your friend to new situations and people. Perhaps they will meet a more compatible best friend. You are not obligated to accept all invitations and it is acceptable to kindly decline some of them. Clingy people panic at having to be alone so instead of traveling together, meet the person at the restaurant or theater and make them wait a little while for you by themselves. This may increase their confidence level in handling being alone. However do not agree to get into the car with them if they are driving. Honestly, you will never get away from them. 
Hopefully the friend will take the hint that you need some space. However if these ideas don’t work, it may be time to seek out the support of a doctor or professional and suggest that they do the same. Unfortunately, you may have to engage them in a heart-to-heart conversation about how they are overstepping their boundaries.  However, if you invite them to be a part of the boundary-setting process, perhaps the two of you can reach an understanding and have healthy friendship after all. 
 [“My clingy friend is getting on my nerves! Friends need a balance of time together and time apart to grow” Irene S. Levine, Ph.D. in The Friendship Doctor. 2010 http://www.psychologytoday.com/blog/the-friendship-doctor/201009/my-clingy-friend-is-getting-my-nerves ]
 [“How to Handle a Clingy Friend: Dealing With an Emotionally Needy Person”, Cherie Burbach. http://friendship.about.com/od/Conflicts With_Friends/a/How-To-Handle-A-Clingy-Friend.htm]
 [Why Are Some People So Clingy?”, Remy Melina. 2011 [http://www.livescience.com/33008-why-are-some-people-so-clingy.html ]
[4 [“10 Ways to Build and Preserve Better Boundaries”, Margarita Tartakovsky, M.S. http://psychcentral.com/lib/10-way-to-build-and-preserve-better-boundaries/0007498 ]