It depends on how important this problem is. Is this part of his nature, something you (and maybe he) both want to change? Or is this something YOU want HIM to change, yet he doesn't see that change as being important enough to engage him in correcting the behavior? Or is this something that he does without thinking, possibly because whatever this is has never been important to him in the past, and he just can't seem to ramp up his attention to it even though he knows it bothers you?
Lots of possibilities here, and they all depend on how you and he each perceive the behavior. When you get right down to it, perhaps it's more a matter of "This may not be that important to YOU, but it's unbelievably important to ME. If you really care for me, you'll make every effort to change this behavior. Will you do this FOR ME? Will you allow me to 'be a nag' about this with you simply because you love me and want to make me happy?" That would pretty much lay it out there for him. You would be making it a "deal breaker" issue, and if he won't allow it to be important for HIM, then the relationship may be on a quick spiral downwards. On the other hand, if he WON'T see how it's important to you or MAKE it important to him, then the relationship may well spiral downwards anyway--it would just take longer.
If it really is NOT a big thing, but you just can't stand it any more, then maybe leaving notes around the house to remind him will help. Tough call from this distance, not knowing more than we know.
Best wishes in this, BMind. You deserve the very best, and I sincerely hope he believes that as well.