I'm in a great marriage, however my wife has never performed oral sex on me, and refuses when I ask. Do you have any ideas on how to change her mind? Locker Talk with her at a time when she's in a good mood, and you have time and privacy to talk. Start by telling her that you love her and you love making love with her, but that you are curious why she won't perform oral sex on you.
If she won't give any reasons then you will have a tough time understanding why she doesn't want to do it. I hope she will tell you openly if she has fears about it, or negative experiences associated with it. Knowing this sort of information can help the two of you get closer, and it may help you understand if she will at some point be ready to have oral sex on you, or if she will never do it because of a trauma associated with it.
If she's just afraid that she won't know what to do, or if she's afraid that she won't like it, then you can reassure her that at first all you'd like is some penile kissing and licking not deep throating! Also, ask her if you can have an "oral love night" in which she should not put her mouth on your penis and you should not put yours on her vulva, but rather you both use your lips and tongues on other parts of each other's bodies.
So that night, first shower together and brush your teeth. Then once you've dried off, kiss each other's necks, suck on each other's toes, nipples, fingers, and lick your way around each other's stomachs and backs. Slowly kiss your way from her neck down her entire body to her toes. The idea is just for you both to be more oral together. Then next time that you have sex, try to integrate more oral activities into love making, again, not necessarily involving genitals, but just more licking, kissing, sucking.
Over time that can evolve into oral sex on your penis and her vulva. If she's wiling to try these things, do not ejaculate if she goes down on you. That should only happen later after she's become more used to giving oral sex. She should not be put in a position in which she feels like she's "servicing" you, but rather, ask her to think of it as just an alternative way to feel good.
If she likes receiving oral sex that will help. Reassure her that sex techniques do not come naturally, and if she's willing to try experimenting with oral techniques, that it could be fun. Never push her head down there, always be clear about what feels good, and talk clearly about the issue of if you'll ejaculate and when and where.
Also, if you can together look at a book that discusses oral sex technique that could help her. My book The Complete Idiot's Guide to Amazing Sex has step-by-step techniques on how she can do it -- but if you don't want to buy my book, then browse for other sex books that are only about oral sex.
Overall, present these ideas to her as if this is a project that you want to work on together as a fun way to spice things up - not something that she "should" do or that is "lacking".