If it’s ball marks on greens you are talking about, I always try to fix not just mine, but any others I see on that green. When a fellow player seems to keep forgetting to look for theirs, I have no problem respectfully reminding them to fix their marks. Sometimes it’s a soft message like “do you have a repair tool or usually use a tee?”. Sometimes I don’t even have to say anything, just walk up and fix theirs while they are standing next to it. The response is usually, “oh, thanks” and then they seem to look for theirs on the following greens. Of course, I can’t go anything about groups in front of us, that’s where golf courses need to be proactive in messaging. Free tools at the starters station, reminders by the starter, posted messages on golf carts, are all helpful ways to minimize the problem. Now if you are referring to fairway divots, that’s a whole different problem that in my opinion, can be solved with a simple rules change.

They need signs stating this (especially on the 1st & 10th tee), Ive seen the free tools and tees and that doesn't seem to help. Marshalls that do their job and not ballhawk would help. Maybe the starter and cashier telling you this when you check in? Print it on the receipt? Dry out greens a bit, never fixed a mark at Bandon or Sand Valley (I know every course is different)!

Like Dale said, a gentle reminder cuts to the nerve!

Regarding ball marks on the greens, if I see that my fellow player did not fix the mark, I will fix it and casually mention it. Also I will bring it to his/her attention like "Here, use my repair tool". Guilt is a great tool for this effort.

Personally, I try fixing my ball mark and any other that I see on the green; same as fairway divots. I fill mine and some in the near vicinity on my ball.

My club has a golf cart pop-up screen message on the first and tenth hole to fix divots and ball marks, as well as another pop-up as to how to repair green ball marks (which is also critical for the health of the green).

We have a 6-hole short course at my club, designed personally by Mr. Dye. It gets trashed weekly by "I can't use the words on the Forums" members that couldn't break an egg with a hockey stick. Sad, but even at top private clubs the etiquette seems to be leaving us. Guys hitting 40-50 short irons to small greens, never fixing 1 ball mark. I have to laugh at them, a few couldn't bend over to fix one if they tried, but that's no excuse....

Don't know if there is a fix for it. Like the other guys have said, subtle reminders drilled into them. I see it as a bit of a reward to fix a ball mark!

I'm happy to fix my ball mark and a couple more, must mean I've hit a decent approach shot.

Another issue I'm starting to see more and more on the greens, the foot draggers. Got to pat down spots in your line and around the hole.

Divots and ball marks seem to be a forever problem. I have 2 sand bottles on my golf car and go thru a bottle and a half every round filling in divots between mine and others nearby. I usually fix 2 or 3 ball marks on every green. Walkers with pull carts should be made to have a sand bottle with them. Obviously, I'm referring to those of us in sun belt climates where you don't replace Bermuda grass divots, but fill them in with sand.

I find that guilt is the best motivation.
I always fix mine and few others, that usually gets my groups attention. Also on the rare occasion that we catch the group in front of us on a tee box, I will ask if anyone has lost a divot tool.. That puts it in their minds..hitemstraight TT

I agree with the others, you have to lead by example. make sure you always repair yours and even the ones that aren't yours

I am like Dale. I repair mine and a few others, especially if some are in my group. I always politely let them know I took care of theirs. They always say thanks and they will most likely take care of theirs in the future. Setting the example is a good start. MADGOLFER, you are a hoot, dude. But whatever works, hakuna matata.

It seems to be a growing problem. With all the youngsters now getting into golf, we might need to require a course in proper care. I do think that the courses need to put up reminders and instructions. I see several where they have a sign at the clubhouse with instruction on proper care, and it would help to have a "fix a divot" reminder at every tee box.
My friends say I'm a bit of an A-hole to golf with because of some of my remarks when playing. Example is when on the green fixing a ball mark I recently told a young player (ha ha, probably in his 30's) to stand back and let the working class fix his mark. Told him as he gets a few years older he'll learn how the human body can actually bend at the waist and fix it's own ball marks.