In the end practice is the only way. I stuggle with this alot myself but this is what I know.
Foreshortening in a nutshell
Simple it is that objects are bigger the closer they get to you.
If you draw a fist being thrown at the viewer you will see two things
(straight on view)
The fist will be big it could make it as big as the characters head although this will give a off cartoony look.
Second we will notice that due to the angle and the increased size where drawing it at it will cover up most of the arm. ( this perferd so you may want to give it a slight angle) none the less it will cover over
third the other side of the body will now need to be smaller because its away from you.
This is a very difficult challenge because it pares up with perspective.
its all a big challenge that what makes it fun and yes in the end all that really matter is you keep practicing with what you know.
I know this is a bad info but I hope it helps in some way. You can easily google search and that will give you better details.