As a sub male wanting his wife to dominate a little more I read through most forums looking for help. I would always aggree that being open and honest is the best way to go, from discussions I had with my wife she felt it very difficult to try and treat me as her slave when to her that is something that a loving couple should not do. Treat others as you would have them treat you type of thing.

It became clear to me that I had to show her two things;
1. She would get benefits to our new found roles
2. When playing I would not be hurt emotionally or hold any grudges for actions taken in that time

The benefits are quite easy to list though it is necessary to keep things as vanilla as possible to start off with. A sensual massage after a hectic day, make sure laundry and household chores are done etc... Make sure that your wife sees that you get some enjoyment out this by saying things like "its nice that since you have "X" done she can relax with a glass of wine".
I found that by offering/asking to paint my wifes toe nails it helped for her to be more comfortable with me at her feet and then sneaking a quick kiss of her toes when they were done again helped with her being comfortable with the idea of simple foot worship.

The hardest part I found was trying to encourage my wife with being more dominant, lets face it how many wives will turn down a massage compared to insulting their loving husbad? With kids around it is very difficult and so alot of thought should go into this but a trick I found was to be descrete and not instantly try and play as Dominatrix and slave.
The greatest difficulty my wife had was feeling comfortable as Domme and knowing what to do to make sure I enjoyed the experience, strangely after several attempts at play her impetous was still on making everything good for me and so always felt awkward in her new role. To combat this we played a few simple games whilst having a drink, yes I know don't mix drink with play but this was not play, purely talking with a drink with a hint of topic discussion towards play.
I created lots of little tickets with humiliating words on, slave... pencil dick... slut... foot slut..., anything I could think of really that I would like to be called or have said during a play time. I then also created tickets with words or phrases that as a loving couple we would often say to each other, love you... etc. All these tickets I put in a bowl which I handed her when we having a drink. The rules were very simple based on a whose line is it anyway game, at random points during our conversation I would point at the bowl. At which point she would then use the word or phrase on the ticket in a setence, something simple at first "you are my slave" and as her confidence grew and with my encouragement and enjoyment, "Kiss my feet foot slut!".
During these conversations I would try to wear less clothing than her, bought myself some mesh pajama pants and the like to get her used to seeing me dressed differently and more exposed. Again with kids this isn't so easy but you get the idea.
Eventually I added to this game was the use of a waterbased marker which she would write something on me with. My wife told me after she first used it that she felt great writing Mistresses Slut on my chest and looking at it for the rest of the night.

These 2 simple little games helped my wife become much more comfortable with the idea of dominating me and now will text message me some of the choice words during the course of my day.

Always try your best do go at your wifes pace, I was always told by my wife that we needed to use baby steps and that eventually we would get there, I just found a way to encourage those steps and tried very hard indeed not to push too much.
Good luck and I hope this helps.