How are you going to tell your bestfriend that she has a really bad body odor?  

how about trying a reverse psycology approache, like asking her/him you smell bad and do you need to wear perfume or cosmetics, try it will work without hurting her feelings

I suggest taking her swimming. In the changing room saying I have a really great deodorant and that I just bought. It was a buy one get one free so I could say to her here you are. Then I could say that she smells so delightful that everyone at work is going to notice. Every day I could check that she is wearing the deodorant. If she didn't wear it I could say to her it is hot weather and it is horrible to feel sticky so it is best to wear this special looking deodorant to avoid having body odor.

I just tell them in private. If its your friend, then they would want you to tell them, before other people smell it. You just don't do it in public for everyone to know their business.

try just approaching her casually without any hesitation just talk to her about it and thats it.You know if she is really your best friend she really mind hearing that but make sure you both are alone when you tell her that.
It is not that she is having some unique problem so she will get embarassed about it, it is just a common problem and there is no big deal.
Just do it.

If you don't wanna embrass her then tell her in private. You can say that she smells a little off then make her smell herself. If it didn't work then just let her be. Someone is bound to smell it and react. You just need to sit tight and relax. You won't be involved. :)

I think that if someone smells they are probably aware of it and don't need coaching.

If she is a good friend, just inquire nicely if she is ill. When she answers just say that usually she smells so pleasant, but today a bit different. I guess there is no real nice way of putting it, but she needs to know.

i got one gal friend..she smell so bad only when it is too hot..ya know when people started to sweat and the odor will be worst..i'm not sure how to tell her..i think she should ask her parents about herself..ya know if there is similar issue to her aboutme...my mum will be the first one to tell me and i would not feel embarrased rather than my friend ask me to wear a deodorant or shower more..it's actually hard to tell her

Just tell her the same way you told us. If you can tell a bunch of total strangers about a problem your best friend has then you can tell her. You want the best for her...why would you NOT tell her???

I will just talk to her one on one and tell her that she smells bad..we are very frank with each other so it's not that difficult. Good thing my best friend doesn't smell bad. You should just talk to her in private about it..it's for her own good anyway..

I will talk to her/ him privately. Then tell the person that Ive been noticing that her deodorant may had let her/ him down. Telling her that I was once like that.. and it embarrassed me. Now, Iam telling her the same thing for her/ him not to be talked about.