Good for Matt, but the key to this kind of relationship is being respectful. Nobody, including women, likes to lose face, and fucking their best friend with such disregard is asking for trouble.

Turn notifications off on your phone when you're with her, give her your full attention. Clean stray hairs off your bed, and throw away things left over by other girls. Destroy the evidence so that her hamster can work. If you greet her at the door with lipstick smeared on your face it's just going to be too much for most chicks, even if they have the tingles.

The truth is LTRs or wives or whoever are important to us, they give us more value than a plate and that's why they have that special status. Treat them with respect and provide the value they're looking for (basically just be a man and give them some quality time) and it will go a long way.

After having experienced this type of relationship a few times, I see that our ancestors had it right. A stable relationship is healthiest when the man gets to fuck around - he basically gets to stay in the constant alpha 'unattainable' state for the duration of the relationship, which is what the woman really wants.