As has been mentioned, clutchless shifting requires easing the load on the gearbox and synchronising the gearspeed for the next gear to go in smoothly. The wider the spread between the gear ratios, the more technique that is required. For this reason, it is better to practice on most bikes in the upper gears and use the clutch for the 1-2 shift. Some of the smaller commuter bikes are different though. They have the lowers gears all close together for better acceleration in traffic and then a big jump to top gear for economy riding away from downtown congestion.
Downshifting can be done without the clutch, but it takes a lot of practive and finesse. You first back off the throttle to put a negativ load on the gearbox. Then, apply a light pressure on the shifter and blip the throttle. The correct amount of throttle, pressure on the shifter, and the speed your foot moves after the transmission disengages have to be just right to get a smooth crunch and jerk free downshift. The trick is to use just enough throttle to run the engine up to the speed it would be running at that ground speed in the next lower gear and shift just quickly enough that the next lower gear pops in just as the engine reaches that speed.
