Leaving a sleeping Pokemon in for as long as possible is another common mistake i see that usually doesn't end well. It might seem like the best option at first to try and burn off sleep turns and wake up as soon as possible, especially with the sleep counter resetting when you switch out, but a sleeping Pokemon is essentially fodder for the opponent to do whatever they please, which is never a good thing, especially with offensive threats such as Quiver Dance Lilligant and Nasty Plot Jynx running around that use sleep moves, as they can become very dangerous very quickly when left unchecked. In my opinion, when playing against a Pokemon with a sleep move, you should try and force the opponent to put the Pokemon you need least on your team to sleep (or your dedicated sleep absorber if you have one, Choiced Sleep Talk Emboar and Escavalier are good examples) and then try and get the sleeping Pokemon out of there as quickly as possible, leaving a sleeping Pokemon in as fodder is never a good idea.