Is it just me or does that sound like the old cream egg adverts?
I normally keep them with the deck. When I store the deck in the card case I put a joker on either side, just incase it gets any knocks. That way, hopefully the jokers get the dent and not the main playing card that happens to be on top. Poor jokers, they rarely get to go to the party.
I use them for practicing card control. I assume the Joker is the "selected card", that way I can see how well I'm controlling it. For example, I can place a Joker on top of the deck, then false overhand shuffle through a few times. If, when I'm finished, the card on top is still the Joker, then I know I did it right.
I just keep them in the deck. I use one of them in an effect - the joker is transformed into a selected card, the illusion is only temporary though and a touch from the spectators finger breaks the spell and its once again the joker
A lot of gimmicked decks don't come with jokers so add them to those. That way when you pull the deck out in performance you can remove the jokers in front of your audience and it works as a nice silent convincer that the deck is normal. Don't make a big deal out of it, just take them out of the pack and chuck them face up on a nearby table without mentioning them.
I use a joker in 'An Incomprehensible Divination' from Royal Road and I always keep them in preparation for learning Simon Aaronson's Prior Commitment (that uses two jokers), bought his book (Try the Impossible) for this one routine, still haven't sat down and learnt it.