You want metaphors that last through time because you want your marriage to last through time. Therefore:
3. The image should be enduring.
Even flowers have cycles. So build an image that includes the cycles, the fragility and the strength that lie behind the blossom.
So if you’re going to talk about a rose, don’t just talk about the blossom. Remember what Linda Ronstadt’s song said: “Love is a rose, you’d better not pick it, it only grows when it’s on the vine. Handful of thorns and you know you’ve missed it, lose your love when you say the word mine.”
Tip: If you’re going to use something like a rose, use the bush, rather than the cut flower. Celebrate the way the flower blossoms and then turns inside during winter. You can find stuff to work with here that will make your wedding ceremony sparkle and your marriage sizzle.