Generally, casual friendships are the best friendships in business, because there is no upkeep required. But with some effort, close friendships can work too. Cheri Swales writes for Excelle with five tips to get it right.
First, set parameters for close friends who might have gotten you a job (or vice versa) in the first place. Nobody should get any special treatment, and you should address each other in the way you would other coworkers. On the flip side, if a friend has really earned a special perk or project, then do not deprive him or her of it just out of fear of showing favoritism. As long as you make it clear that you are not indulging in clique behavior, you will be fine. Additionally, watch out for two-faces; you are not sure who can be trusted not to blab about or misuse sensitive information at first, so feed it to colleagues piecemeal. Lastly, remember that you can make more friends outside the office too. You never know who might give you your next leg up in work or in life.
You can read the full article here: http://excelle.monster.com/benefits/articles/1864-5-tips-for-making-office-friendships-work