An enema doesn't strike me as the sort of thing that one can just dive into. There are contraptions, and devices, and rules to follow and safety to keep in mind, not to mention you're introducing a substance to the human body (even though it's just salt water).
Just coming out and saying "hey, I want you to give me an enema" is about as shocking as saying "hey, I want to force feed you through a tube".
*crickets chirping*
Okay, perhaps that's not shocking to the people here either. (*whew* - tough crowd!)
Anyway, it strikes me as the kind of thing that one would want to EASE into. (I mean the SUBJECT should be eased into, we all know that the tube is eased into you as well, but that comes later!)
First of all, COMMUNICATE with her. It all depends on your relationship and the degree to which you talk about sex. But talk to her about your sex life and ask HER what YOU can do FOR HER to make the experience better.
Start by addressing her needs and wants; that will definitely build trust. If you do a good job and she's a smart cookie, eventually she'll ask the same thing - what can she do for you. And then, start SLOWLY. Don't just dive in because you don't want to scare her off.
Reassure her that she's wonderful as she is and that she doesn't need to do things that she doesn't want to do, that you'll love her no matter what.
And reassure her that your sex life isn't bad now - that's it is wonderful and that you're not hoping for new things to make the sex good, but that you're just hoping to try new things to keep your sex life fresh. Again, it's because you LOVE her and you love the intimacy with her.
And then, best of all, make her think it was her idea. The point of this whole exercise isn't so you can say "hey look what I got my woman to do", it's ultimately so that say "my wife and I have a great sex life and we meet each others' needs and wants". So nobody cares who came up with the idea, but if she comes to the conclusion herself to do it for you, then she'll be a LOT more committed to it.
So, perhaps play dumb; if she asks what can she do for you, say, "hmmm, I don't know, let's get some ideas" and surf the internet with her a bit. Use it like a catalog: pictures of girls with pig tails and glasses giving BJ's? Hmmm..that might be nice, so you try that a time or two.
Surf the internet with her some more: movie clips of women tying their men up? Hmmm..that's interesting, let's try a bit of that.
A time or two later, and by now you've made the internet surfing and trying new things that you find a habit that you two share: Women dressed as nurses with rubber gloves giving hand jobs? Hmmm..that IS interesting....let's try that...
And still later: Women dressed as nurses tying their men down and giving them ENEMAS? WHAT? WHY? Then you poke around a bit more and "find" sites like this one, that bestow the pleasures of erotic enemas. And the two of you learn about it together. And you conclude "hmmm...that's interesting. You know, I might actually like to try that."
Okay, long winded, I know. Sorry about that, but I think you get the picture: reassure her of your love, cater to her needs first, learn new things together and you'll likely get her buy-in. A few months of work, and you're in for a lifetime of happiness. Above all else, observe and respect her boundaries. If she says "no", then "no" it is and let it drop. Move on to other 'games' and either you'll find something better because it's something you share, or she'll warm up to it after trying other new things.
Good luck.