The best way to get ideas for writing really funny best man speeches would be to download sample best man speeches and other materials and start reading them. I have just suggested you a good resource for downloading speech samples as well as templates but there are several others as well.
While giving speeches that you expect your audience to laugh, you need to focus on the content and also the delivery. If you think the anecdote or a story you are thinking of sharing about the groom is funny then it will probably be funny. But you must make sure to bring it up in an appropriate part of your speech.