Please note: I am a submissive, and I am writing this from that perspective. I've never switched roles, so this is based solely on what I've observed. I'm also not saying that you don't know/do some of these things. I don't know you, so take from this what works and ignore the rest of it. Nothing works for everyone.

You've had some excellent advice, so I just wanted to offer a up a few quick things to consider (and I may be reiterating some of what's already been said- I apologize for that.):

Make sure you're on the same page with your submissive. Know her limits. Know your limits (and I don't think I've ever spoken to a Dom without limits of His/Her own). Understand what she is getting out of it, and understand what it is you are getting out of it. Listen to what she wants and decide where you are going with it. Sometimes the route to getting there isn't linear enough to fit in a spreadsheet, but the idea is to be organized within yourself. You can't be in control if you're blindly forging ahead, because then the situation itself is in control, carrying you both along with it. Know where you're going, but don't tell her. Learn to be a control freak, but not an asshole. Be aware of her mindset at all times. It is your job to protect her from actual harm, and that includes emotional/mental harm. It is also your job to push her buttons. Make sure she is aware of her responsibilities. If you change the rules in the middle of something, make sure it was an intentional thing you did to throw her off-guard, and that there's a safety net in place in case she doesn't respond well. Analyze everything. The more you know about her, the more you can get from her. (I'm not talking about blackmail here, I'm talking about knowing that she hates the idea of anal sex, but probing to see if she would be willing to explore it for you, as an example.) Know what she wants, even when she doesn't.

I suppose it all boils down to the old adage: Knowledge is power.

That's what worked for me. Good luck!

Also, as an aside, it is entirely possible to be submissive without being trained. That's what everyone who is submissive was prior to training- in other words, it's not something that can just be imprinted onto a vanilla person. Submission is from within; it's not a rank in a lifestyle.