I've been creating, working on and playing several shows lately. Several close up magic shows. Not just a bunch of tricks strung together for 5 minutes, but real shows. One is 90 minutes in length and one is 70 minutes and another one is 50 minutes.
I realized that a running gag really helps tying the show together. More so than the over arcing theme, atmosphere and the premise. Sad thing actually.
Maybe I should specify it a little more. I'm not talking about a comedic running gag, but a magic running gag.
Some of you may know the "Water from India" gag, where water appears from a bowl which is emptied and then set aside. Later it has more water and everything is repeated. I rarely see a good magic running gag. A running gag should have a payoff. In the Water from India gag the usual payoff is that the second bowl the water is poured in is tipped over in the end, to reveal no water or confetti.
Here is a simple example you can use yourself: Have a card picked in the beginning, then signed then lose it in the deck. Say that you will make the card go to your left pocket. You reach into your left pocket, but it's the wrong card. Everybody has a laugh, but you don't give up. You claim to try again... later. Then continue with your show. Later you reach for the pocket and again produce the wrong card... you see where this is going? Just keep doing this to tie together your act. In the end you find the card in a spectacular fashion of course.
Or borrow a bill in the beginning. A large bill. Fold it a few times then unfold it and it is a smaller bill now. Offer to reverse it.... but instead it is even smaller. Put the bill away telling your spectator to come back to it. And you do come back to it. A little bit later. But it gets even smaller. Eventually the bill becomes a coin and even the coins gets smaller. In the end you produce the first bill and give the coin and the bill to the spectator. All of that has great comedy potential and great audience interaction.
So you see basically a running gag could be just a multiphased routine spliced apart by time. Try it and improve your magic show.