So you have to make 12 points for each age and this is modified by +1 for each city, +5 for each previous golden and -5 for each dark.
Too many golden ages and cities will make you collapse at the end, I guess that is why it is rise and fall.
Are you sure you are scraping everything you can? I have only played a couple of long games but I have found going for normal ages is just fine and then when your cities are large getting a dark makes little difference as your large cities will not suffer loyalty.
There are things like launching landing on the moon is well worth doing as not only will it give you an era point but give you good culture for a bit of you do it right.
Great Generals and admirals are another... you can even save some up hidden in a corner of your empire and then wheel them out for a skirmish at the end where you take no cities but kill the odd unit which will give you +1 era for each admiral and general.
Bodyguard of lies, Wish you were here and Sky and Stars will all give plenty of era points if you concentrate on them
Placing a city near a natural wonder, settling a new city in bad lands, getting great people (run projects if need be), levy a CS military, finish spaceship components, and a trading post in a civ you have never had one with all will help and I just do not see an issue unless you have had too many goldens and have a massive empire... in which case you probably deserve to be dark as you should have won by now...
Bottom line... if you want to play a long game for fun, being too large and too successful early will come back and bite you later. If you want to avoid a late dark you need to concentrate the era before the last on just getting normal.
overshooting your Golden Ages
Not 100% sure this will help their problem because late in the game most era scores are gone... the overshooting is about holding back era score from one era to the next and that late in the game there is maybe the odd 1 point that would be valid but not large amounts... still a fair point but just not big.