From personal experience, I have yet to meet a woman who is not into a little 'light bondage' and spanking. Silk scarves on the bed posts, blindfolds etc. So this may be a place to start. Unfortunately, it is also my personal experience that the majority of (vanilla) women seem to want to be the one tied and spanked so you may instead find out about her latent sub side

I'd go with the subconscious service things already mentioned. Do jobs for her without her asking you, give her massages and buy her treats. Work up to it slowly. During sex, try a little foot worship and make sure that her needs are met first. Tell her that you love it when she takes charge in the bedroom, let her take dominant positions and so on. Take it as far as you feel comfortable with and a little more each time but slowly...