What happens to Mythos, Trauma, Keeper Action cards that have been used once?
For Mythos Cards (Page 16):


  After playing a Mythos card, the keeper discards it to a faceup pile 
  adjacent to the Mythos deck. If the Mythos deck runs out of cards, this 
  discard pile is shuffled to create a new Mythos deck.


For Trauma Cards (Page 25):


  Each investigator may have a maximum of one injury and one 
  insanity at any given time. If he receives a second card of the 
  same type, the original card is discarded first.


Additionally, some injuries/insanities say discard immediately.

Under Component Limitations (Page 25):


  Decks of Cards: If a deck of cards runs out, the players 
  shuffle the discard pile to create a new deck. The only 
  exception is the Event deck, which always ends the game after 
  the last card is drawn (as listed on the card).


For Keeper actions (Page 10):


  During this step, the keeper may resolve any number of these 
  Keeper Action cards that he wishes (threat permitting). These 
  actions are resolved one at a time by spending (discarding) the 
  appropriate amount of threat (printed on the upper left corner of 
  the Keeper Action card)


So Keeper Action cards don't get discarded ever, just the threat tokens. In fact, some keeper cards can be used multiple times in the same turn.