Avoid clashing patterns and choose something that complements each other
This is pretty straight forward, If you were wanting to wear a floral dress, then choose something for him that is neutral and can complement it. You want to avoid wearing patterns that are completely different, because even though they might look awesome on their own, they do the opposite effect when standing next to each other.
Also, I know that being super matchy matchy was all the rage in the 90s, but now you want to wear something that represents you! Bring your own style and personality to it! Then from there you can work to complement what your fiancé is wearing.