been trying this in my garden (never really done it before!) and i learned something that might help. bar with me, itll help it make sense 
you know when you go bowling, you see the crap players letting go of the ball way too high, it gains a little height then lands straight on the alley with a BLAM! and a load of speed is lost.
Then you see the good players getting the ball so low to the floor that theres practically no impact, since they let go as close to the floor as possible, so the angle of attack to the floor is really low and the ball hasnt had a chance to start speeding towards the floor.
If you imagine youre about to push off with your leg, push off later than usual, so that your body is unusually lower to the floor and leant forwards a lot more than what youre used to, then when you hit the floor, its with forwards momentum, not plummeting down. this way, youre already close to the floor and when youre pushing off, youre pushing more forwards than upwards, plus you dont seem to smack your knees on the floor as easily, as well as not getting winded hard!
just make sure you lean so much youre beyond the point of no return when you push off with your foot.
make sure the hand holding the marker has its palm is facing upwards, then the reflex of putting your hands out when you get close to the floor wont make you plug the barrel.
if you roll like i do sometimes, try putting your elbow out a little more.
like i said, im not well practised, but the mistakes i made might help you a little! the problem im having is keeping speed while preparing to dive so low, i cant run so fast when im getting so low to the floor in anticipation, but i guess that comes with flexibility.
you know when you go bowling, you see the crap players letting go of the ball way too high, it gains a little height then lands straight on the alley with a BLAM! and a load of speed is lost.
Then you see the good players getting the ball so low to the floor that theres practically no impact, since they let go as close to the floor as possible, so the angle of attack to the floor is really low and the ball hasnt had a chance to start speeding towards the floor.
If you imagine youre about to push off with your leg, push off later than usual, so that your body is unusually lower to the floor and leant forwards a lot more than what youre used to, then when you hit the floor, its with forwards momentum, not plummeting down. this way, youre already close to the floor and when youre pushing off, youre pushing more forwards than upwards, plus you dont seem to smack your knees on the floor as easily, as well as not getting winded hard!
just make sure you lean so much youre beyond the point of no return when you push off with your foot.
make sure the hand holding the marker has its palm is facing upwards, then the reflex of putting your hands out when you get close to the floor wont make you plug the barrel.
if you roll like i do sometimes, try putting your elbow out a little more.
like i said, im not well practised, but the mistakes i made might help you a little! the problem im having is keeping speed while preparing to dive so low, i cant run so fast when im getting so low to the floor in anticipation, but i guess that comes with flexibility.