:: what not to say ::

but what if she says no?!
good question. as the boy scouts say, it’s best to be ready for whatever life throws at you – or something like that – and who knows if Life will be in the form of Your Sweet, and there’s an airborne antique vase aimed at your noggin? 
several things should be kept in mind, if such unpleasant things were to occur. (note: passive voice is excellent for keeping blame generalized.)

~ as already noted, Do Not Treat Her Refusal As A Joke. bow in quiet politeness, appropriate solemnity, and manful dignity. “this word from her will silence you on the subject forever”; then exit. do not immediately propose to her best friend.
~ vow to be there for her (i.e. the tiernan o’malley) but do not kiss her farewell, even if she is in a field of violets or has just completed a tennis match (i.e. the george emerson). 
~ promise that even in light of this event, of her friends “she can count you as one of her best”.  
~ to “procure a lock of her hair” is allowed, if it is to be kept around your neck for burial, and if your burial is within the week.
~ For the sake of the Loved One, conquer these Feelings of Bitterness and become Magnanimous.

in sum: treat her with respect and deference; be appropriately heart-melting but not overly dramatic; don’t go for revenge because of your shattered heart. jerky heart never won fair lady. and, whatever you do, don’t explode the opera house.