love the rings!

and i like cariad's idea as well...very unique

or..

you could take the first letter of a few words that define her love/service to you...as well as yours to her and put them on the rings...

so her ring has the letters S O L (submission/slave, obedience, loyalty/love) just as an example...and yours could have D L T S (Dominance/Discipline, loyalty/love, training, strength....)

just examples...you don't have to use all of those words and each letter could mean something different or have multiple meanings to you and her...but it would be nice for her to look down at the rings and always be reminded of what you are giving and sharing with each other in your relationship.