Sometimes it's better to aim stickies by literally shooting areas and not people.
Watching demo reviews is great, looking over your own demos is really good.
Practicing pipes can be important but doing exclusively that on soap dm can make you worse at stickies (which are a lot more important) so I wouldn't recommend it all the time.
Termo's i52 povs give you a good idea of how to play and position yourself on demo but also are a good indicator of a good way to aim on demo (although it's not the only one) as his dm is really really good.
Practicing with a team is important as demo is hugely influenced by how people play around him and being consistently with the same players lets you adapt a good playstyle.
Think about ammo conservation, when to use each of your weapons, what targets you should be focusing, when to call focus etc. Try and mainly focus forwards at the chokes and don't focus on enemy players behind you when there's other players to deal with it and people to damage in front, you're the no 1 class who shouldn't get distracted. Think about whether you should be taking ubers or if your med can solo your pocket and you can stay out. Also think about how many heals/how much spam you want to be taking.
Remember, demo is a hard class and one that can't do much in dm situations a lot of the time so don't get frustrated when you get buttfucked by out of position players when you're reloading sometimes.