For example:

If you have a dog, say, "You have to put the seat down, or Fido will start drinking out of it!" The same goes for a cat in the house. You can then proceed to discuss how that dog or cat then licks their face out of affection... and where has that mouth been?
Inform your male spouse or partner that the children have taken to fishing in the toilet every time they find the seat up. And they're using his ties as rods, with his cufflinks as bait.;
, If you keep saying it to them and repeating it over and over, it should (hopefully) get through the layer of hair and into their thought processes.

Leave a sign, sticker or note on the toilet with a reminder to put down the toilet seat. These could say such things as: "Put Me Down" and "In this family we do the toilet seats/lids DOWN."
Find a scientific article that explains just how unhygienic it is to leave the toilet lid up when flushing. There is plenty of evidence out there that it's a lot safer to shut the lid on the germs, including preventing the spread of winter vomiting bugs.(It also helps to inform the men that everyone has to put the entire lid down, not just the seat, so it's not singling out any particular users, but the entire household must cooperate in the war on germs.)
Ask them why the toilet has a lid. Is it perhaps for amusement? For decorative effect? For balance? Or, perhaps it's because it keeps out odors, viruses, and grime? A reminder of the fact that the toilet has a lid suggests that said lid could do with being used.
 Set an example for them, if you do it maybe they will too. Monkey see, monkey do, right?, If none of the above ideas work, use technology:

Alarm: Purchase a Tip Alert(R) toilet seat alarm. It should solve the problem. This alarm works by letting off a wolf whistle with such comments as "Down boy!" when the seat is left tilted up for too long.
Install a seat that automatically closes.
Find a really good reason for why the men in the household should put the seat down.
Pester the men of the household about the toilet seat endlessly.
Put the toilet seat down yourself.
Try the technological solutions.
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