My best friend is my boss and has been for the last year and a half.
We have been friends for 20 years. We met on the job and worked side by side as equals. About 7 years ago, she left our company to join the government. Because she values my work, as soon as an opening came up and she was in a position to hire, she hired me into her office.
She is my direct supervisor. There have been no problems whatsoever because both of us are mature and sane individuals.
As a friend, I would NEVER put her in any position to have to treat me differently or show any sort of favoritism. Both of us are very sensitive to our relationship and how it appears in the office. I don't ask more of her or for any special treatment than I would any other boss and YOUR friend needs to do the same.
In return, I am treated just as any other coworker. In fact, sometimes I might almost be "punished" for being her friend because we try SO HARD not to show any sort of favoritism that sometimes she might not do things for me that she normally would because the issue.
Overall though, it has not been that difficult. I will say that the situation has to work BOTH ways and your friends are going to have to do their part and not have unrealistic expectations of you. There is going to be the "friendship" and then there is going to be "business" and they have to expect that both of you will be very objective and impartial when it comes to the business.
As I said, I believe it can be done with mature, sane people but, I happen to know there aren't always a lot of those out there.

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Good luck and congratulations on your promotion!