When I raced 125's years ago, clutchless shifting was almost mandatory to maintain momentum. Once you learn the exact technique, you pose no harm of accelerated wear on the shift dogs.
I race 125cc shifter karts with a CR125 powerplant, and with karts, you only use the hand clutch to stop and start. Like others commented you simply make one perfectly synchronized motion: unload the gearbox by abruptly letting off throttle, abruptly shift into the next gear, back hard on the gas. Once you have the technique dialed in, you will see almost no rpm loss, and the actuall shift will feel very smooth. If the shift feels notchy, or you feel resistance, your not unloading the gearbox correctly (I am also shifting my kart consistantly at 12,300 rpm's), and kart racers go years without a tranny rebuild
