People who struggle with masturbation while married are often being denied their marital rights in bed and that must be corrected.
Being candid on this one:
I struggle with masturbation in marriage on account of my wife having a much lower libido than myself. Simply demanding my marital right would be tantamount to rape, and it in no way increases the intimacy of the relationship.
As far as I can see, there really isn't a workable solution for this situation other than to just go hardcore and suffer through things... which is fairly difficult, as unlike the monastic or the teenager, extinguishing the sex drive entirely is undesirable and so methods useful to the monk and teen are useless.
Pressing through suffering by the help of and reliance on god's grace, though, is exactly what needs to be done. Marriages are founded on love, and christ says that no man has a greater love than he who would lay down his life for his friends. Let us consider then, our suffering as a type of laying down our lives for our friends, and follow christ's example in this. Christ does not derive methodologies for alleviating the suffering on the cross, but does it plainly, and so too must we plainly suffer in our own crucifixion against these temptations, given endurance to do so by the grace of god, such that we can elevate our love to that variety which christ calls the highest.
This is one manner by which paul calls marriage a mystery of christ and the church in ephesians 5, for through the comprehension of these facts, we can more fully understand christ, and this understanding is certainly a grace given to us by god through the sacrament of holy matrimony.
avoid masturbation by HAVING SEX WITH A REAL PERSON AS GOD INTENDED
well, that's good advice on paper and I've seen it echoed in many communities, but it honestly sounds like the sort of advice that comes from someone who isn't actually in a sexually active marriage. It reminds me of the misguided question a single friend of my wife's asked her when we were newly married: "what's it like to have sex everyday", to which we both laughed.
It doesn't work that way -- people have different levels of needs, different capabilities for fulfilling their partners needs, varying work schedules, varying sleep/wake cycles, different times that they're aroused or even capable of becoming aroused. And this is before mentioning impediments like sickness, differing levels of physical health, etc etc.
In short, your partner will not always be able to satisfy your sexual needs, and if you take this "have sex instead of masturbating" rhetoric to it's utmost ends, you'll end up resenting your partner out of their inability to satisfy you on demand and ultimately you'll end up committing adultery.
Not masturbating is solved like any other sin: following christ in taking up our crosses and frequenting the sacraments, ever aware of god's grace being the source of our ability to do good. When we fall, let it be that god lets us fall to keep us humble and fully aware of the fact that we are incapable of perfection without his grace. It is most humiliating to confess this sin, and so let it humiliate us to the position of bowing before god's primacy in all things over any trust we have in our own abilities apart from god's grace.
just sorta my thoughts on it... sorry if it's a bit rambly.