You should go over a detailed list of all that the bride and groom should and should not do over the course of their marriage.
Then, make them solemnly affirm that they will NEVER do X, Y and Z-- FOREVER!
After all, you have an obligation to speak someone else's personal convictions into their lives! They asked for it when they asked for you!
Now is your chance to tell it like it is!
Now is NOT the time to COMPROMISE!
Have them sign the pledge in their own blood:
"I will never tell my wife I don't like what she cooked."
"I will never make my husband do the laundry."
"I will never beat my parent in-laws at any competitive game or conversation."
And any other non-scriptural prohibitions you can think of that might make their marriage a long-lasting marriage that lines up perfectly with other people's wisdom...