Online you should play shorter sessions and less tables. Helps for sure to focus more what happens at the tables. I also find it very useful to make notes.
When you notice that your concentration is slipping and you are making more and more mistakes, stop.
Of course, this only applies to cash games.
If you run deep in a tourney and getting tired, nothing you can do.
Years ago I tried to play 20 tables 6 max at PS. I played horrible. No time for taking notes or paying attention to what happens at the tables. I reduced it to 6 tables and it worked great.
Of course there were guys who could play 24 tables profitably like Nanonoko or Dustin Schmitt. I think those were rather exceptions.
I found "
boku" to be the most extreme, playing 50 or more sng's at once.