If you pull on tne shifter as hard as you can under full power , you will not pull it out of gear , so the basics are to pull/ push hard on the shifter , kick the clutch pedal 2/3 of the way down & slide off the side of the pedal allowing it to re-engage on it`s own whil enever lifting on the gas pedal!!
I highly recommend a non diaphragm clutch & a Rev limiter
You can also shift without using the clutch , all the truck drivers do it but it is slow , you need to match the rpm in the different gear to slide them together