In addition to improving your throw, keeping your yoyo sleeping longer during tricks is also a lot about keeping the yoyo straight and in plane while you are doing tricks. If the yoyo is moving forward and backward a lot while you are moving it around, that takes a lot of spin off the yoyo. If the yoyo is angled a bit relative to the plane of your tricks or starts tilting, that also hurts spin time. If the yoyo does start to angle or tilt, you can correct it by angling the string against the walls of the yoyo to re-orient the yoyo so it is spinning in the right plane again. You might have to experiment with this to see how pushing on the walls of the yoyo with the string affects the yoyo, but once you get it down, it’s not very hard to correct the yoyo so it is spinning fairly straight. And once you work on getting your throw good and straight and keeping the yoyo moving in plane, you shouldn’t have to correct it that much.
Most people will stop and adjust their string tension with a specific trick designed to do that when they notice their tension is off, but it is possible to make small string tension adjustments in the middle of play. The natural tendency is for the string to slowly tighten (or loosen if you are left-handed) during play, but there are tricks that will have the opposite effect. Tricks that move the yoyo or string in the opposite direction of the initial throw generally help keep the string tension closer to neutral. So tricks like Follow or Jade Whip, or lacerations that whip the string in the opposite direction of normal lacerations, or even just counter-clockwise pinwheels (clockwise if you are left-handed). Mixing those in with your usual tricks can help keep the tension neutral longer. Tension-adjusting tricks like jhb was talking about are not really any trouble once you learn to do them, though, so it’s not that big a deal for most people.