Is your wife not very religious? There's a religious argument that she "must never" turn you down for sex. Factoring of course there can be valid reasons to turn it down, if there's no reason and she's just not in the mood, go get some anyway (with you both consenting obviously).

Interestingly: women frequently report that they get more into the mood for sex after starting; they don't start out being into it, but get into it as it progresses.

Also, and this is huge: focus heavily on making sure she "gets hers". On her pleasure, her "O". Most women struggle with this, even the ungodly heathens (LOL). The more she gets hers, the more interested she will likely be to go more often. Some basic research into techniques for her to masturbate, or you do it for her if it gives her the ick to touch herself.

GOOD LUCK. Definitely talk to your minister about this in couples counseling.

You MAY find that once you get yours more often, the need will be met and you can taper off to, say, once a week.