Well, the unfortunate answer (the one you don't want to hear) is if you're doing monogamy, and you're taking a girlfriend for a stretch of time and not sleeping with other women, your skills will inevitably rust.
You can slow the rusting and put a floor to it if you maintain an active social life and continue to flirt a lot with other women. The farther you take these flirtations, the less rust you'll accumualte, and the slower it'll build up. But of course the flipside of that is heavy flirtation with other girls is a risk to your relationship if you're monogamous, and it might not be feasible if you mostly do social circle and your girl comes from or is plugged into your circle.
Past a certain skill level, it gets to be like riding a bike, and you don't ever accumulate so much rust you lose it for long. I've seen a lot of guys who were very good, who went into monogamous relationships for 5 or 10 or 15 years, then went back to the game, and usually these guys have some early successes right out of the gate (a few rapid lays), followed by a stretch of shaking off rust, followed by them hitting peaks of better results than they ever got before their long relationships.
The best advice if you're worried about keeping rust off is "Try to reach a level you're fully satisfied to be at with girls before you settle into any kind of monogamous commitment." If you can do that, the rusting won't be too bad, usually, and it's easy-ish to shake off later.
There's one other element of this, and that is how fully you commit to the monogamous relationship. If you're just all-in, with no thoughts to other women, you'll rust more and it'll be harder to bounce back later. If you keep a part of you that says "It wouldn't be so bad to be single again", you will think about other women more, probably talk to them more, and won't rust as much or as quick, and will handle it better if/when the relationship ends.
Which is not to say you shouldn't go all-in. Maybe you should. Depends wha you're after in life.