I've found that when I hit a royal (usually for $1,000) I lose all interest in playing even one more hand. More often than not, I'm hungry then, so I seek out the best restaurant in the casino and chow down. Often, I'll order dessert, which I wouldn't do normally.
Also, if I do keep playing, I might drop down in denomination. That way, I can keep playing without blowing back a substantial portion of my big win. For instance, if I hit 2222 on a .25 Deuces machine, I might cash out and put $100 in the same or another machine--and play dimes or even nickels.
From a logical standpoint, whatever reason you have for playing shouldn't be affected by whether you're ahead or behind at the moment--but there's no worse feeling than posting a big win and then blowing it all back.
A friend of mine used to, after a big hit, immediately run out and buy something for his wife. That preserved his win, got him away from the machine, and scored big diplomatic points. She wasn't going to object to his playing!