The problem with trying to follow that advice, of alternating long and short tricks, is the one you've just run into... what constitutes long, and what constitutes short. So while it sounds like good advice, don't use it to plan your entire show. Instead, liven up your long routines with short bits of magical excitement.
Do you recall the first Indiana Jones movie (Raiders of the Lost Ark)? It set a new precedent in movies in that something exciting happened at least once every minute. Today, most action flicks follow that rule. Excitement every minute.
So in your long routine, if you have little things appearing and disappearing, flashes of fire, quick bits of levitation, color changes, etc., you are following the same principle.
Consider the standard Kid's Show routine where you make a blank coloring book suddenly have black and white pictures and then the pictures become colored in. Add some elements, like the multiplying crayons, the vanishing crayons, the rainbow silk that loses its colors (as they transfer to the book) and you have now taken a long (and somewhat predictable) routine and filled it with short moments of exciting magic.