BM speeches are very formulaic and so the only real way to make it different is to make it personal to the bride and groom, their families and friends, without being rude, offensive and without spouting off humorous tired old jokes from a compendium.
I remember one speech I heard was when the BM said, �I always thought ginger people smelled of wee until I met Jane� That got a few embarrassed laughs but the brides family were really miffed!
You are there ultimately to make people laugh at the groom�s expense, but in a nice compassionate and friendly way. And you can do this any memorable way you wish. People will be coming up to you afterwards to comment so be careful not to inadvertently insult auntie Doris.