It might be prudent to hold onto similar cards until you can use them effectively or until you are positive they won't help your opponent, for example, if you observe an opponent taking cards from the discard pile that are almost rummy koready to form a sequence. On the other hand, choosing cards from the draw or discard pile also needs to be done carefully. Make sure the card you choose from the discard pile complements your present plan and advances your efforts to create legitimate combinations. It could be time to review your hand & think about discarding any cards that don't help you reach your overall objective if you find yourself holding onto an excessive number of unrelated cards.