regarding practice techniques you have to isolate the different components.
first you have to be able to upshift and downshift smoothly and quickly while in a straight line...
set an arbitrary speed, say 40mph...and shift back and forth between the same two gears...and make sure you stay rock steady at ~40. since you seem to be familiar with the "feed the clutch out slowly" method of downshifting...
you should practice blipping on the downshifts. smooth is the key, you don't want any hiccups in the bike whatsoever.
next you should be able to smoothly transition between brakes and throttle. in order to be super smooth you can remain on the throttle even when you initially start braking...and you should still be braking when you initially start rolling back on the throttle. not only is this a smooth way of transitioning...it will also get you accustomed to using the brakes and the throttle at the same time.
you will want to analyze why you are letting off on the brakes when you are blipping the throttle. it's hard to visualize your exact problem without seeing you in person.
check the position of your brake lever. check your foream arm/wrist/hand positions too...they should be in a straight line (no ugly bends).
you shouldn't really be grabbing, holding, or adding weight to any part of your hands either.
you can blip with your palm (so that all fingers can be braking)...and you can blip with your outer fingers while the first two are doing the braking...and you could do both...
if you think about it, just keeping the same amount of throttle while braking will already cause the revs to rise when you pull in the clutch lever (let alone blipping).
this is something you can practice with the bike off and not running.
the key is practicing slowly and methodically when your mind is at ease. not much is going to happen if you practice doing it all at once at speed...
whatever the case, do NOT go out and buy a slipper clutch. that would just be a bandaid, and not a cure.
edit: two more points...
1. when braking while downshifitng your priority is braking. don't sacrifice your braking trying to blip at the same time.
2. ideally you want to raise the engine revs the precise amount for the next gear...
(for example, if 40mph is 8k in second, and 10k in first, you want those engine revs to be at 10k when you fully release the clutch lever)
however, if you're going to err...do so on the side of blipping too much rather than too little. too little gets the rear wheel/engine VERY unhappy.