How to practice being a manager?

If volunteering for a non-profit isn't your thing, okay. In its defense, I will say that volunteering can take many forms, from a few hours each day to a few hours each month, and there are plenty of deserving groups who could use help. But if that really doesn't do it for you, my advice is to start your own business. I'm serious.
Don't quit your day job, don't go part-time, don't go nuts. Take some skill you have: programing, design, accounting, and become a consultant. Become a shade-tree mechanic. Tutor high school students in math. Try to bring in $50/month. Once you meet that goal, see if you can bring in $200/month.
Don't call yourself CEO. Don't print up business cards or take out huge ads. Put up a few fliers, buy a few Google Ads, donate "Two hours of technical writing" to a charity auction in your neighborhood. Run your own business a few hours each month and you'll be amazed at how much it helps you during your 9-5.
I did this years ago to give myself some daylight when I felt trapped in a dead-end individual contributor role at a huge publisher. I started my own little publishing company. Now I have three books, two contract employees, and there are two authors right now criss-crossing the country selling the books I helped them bring to market. Do I still have a 9-5? Sure. And I'm obedient and a good team member. But I also have insight into product development, marketing, and project management. It would have taken me a decade to get that kind of practice from my day job.

AFMOFFA, your suggestion is great. 
My reading of Ryan's post is that he is looking for ways to practice specific skills, not ways to play more games.
Since Ryan mentioned O3s.. I think that you could ask friends to do role-play O3s with you.  Or take a snippet of an O3,  and practice responding in different ways on your own?  Script an O3 and film yourself delivering to see if you notice behavioral quirks that might make it less effective for your directs.
Back to practicing any skill, you find a way to exclude the things you don't want to practice so you can focus specifically on what you do need work on. 
 

Filming/recording myself doing an O3 is an awesome idea.  I could do the same thing during my team meeting.  Then watch/listen to it afterwards and try to improve.
AFMOFFA, I agree and I did the same thing when I was an individual contributor.  It certainly helped me get into management but like AKINSGRE said, I have *lots* of opportunities both @ work and at the volunteer oppts that I have to do more leadership.  The problem is that right now I'm doing 12-14 hours of game playing a day at various levels (volunteer, work) but what I want is to hone certain skills.
In (most) other professions, you practice a sub-skill, then look to see how the expert did it, learn what you're doing wrong and then practice again to get it right.   I feel like all I do is game play, which doesn't really hone any skill. 
I think of teachers who work for 25 years, and unfortunately more often than not, end up as worse teachers than when they started.  Just showing up for work is a terrible way to accelerate improvement.
Ryan

the video cameras are for the practice one-on-ones, right? Because if you're my boss, and you set up a camcorder for my weekly one-on-one, I am going to sprint, Carl Lewis-style, to the Human Resources office. I think everyone's just talking about play-acting o-o-o's, but I just want to underscore the point.
Could you practice with a peer, perhaps in another division? Meet for lunch at his desk once a week, and you'd each role-play a challenging one-on-one. It'd be like Venus and Serena Williams; one of the reasons they're both so good is they're the only world-ranked tennis pros who get to play each other for practice. You and your peer would get really good, and you'd (indirectly) get to know each other's team.
 

Yup, you're right.  I have a webcam built into my laptop, so I could possibly turn it on in a 1-1 but I have no intention of filming/recording the person in the 1-1.  And you're right,  I'd freak out too if my boss asked me to film our 1-1.  If I don't tell them and they see me watching it later I'd lose far more trust than I'd gain in mgmt experience.  That's too bad.
Now I need to hunt for a partner to do practice doing O3s with.  Thanks for pointing that out.
Ryan

You know violinist also Learn, improve as they Play? They do not start as being first violinists, they get there as they Play and only the best get there. And while they are in the third road of the orchestra they are allowed to make some mistakes noone will really notice. Of course you will become a better manager, but you are a manager already, and if you are concern and focus on it you are doing well. I find that the big shift from comming from a technical profession to Management is that it changes the focus from things, equations, computers to people. The only thing that i think can complement manager-tools in management is understanding people better. You can read books on psicology, the different times of communication skills, the disc model, crucial conversations,....or just observe more the people that work for you. You say you are not improving, ask yourself why you think you are not improving. Are you not getting the results you want or for you it is difficult to do the o3s? If it is the first then focus more on the people and the tasks you want them to do, it is the second focus on yourself, perhaps there is something you must addapt, change on yourself to enjoy the ride more.

Think of working, Managing people as debugging a very badlly documented software you have not written yourself but is having to modify. You write one line of code which is obvious to you but it does something else, you them have to them find out where the bug is. You may think you are not advancing but you are, a lot slower though then if you had written the code yourself, and it will never be perfect.

Nara