improving multithreading skills

Practice and in particular practice under pressure.
You can learn the theory and APIs for multithreading for fun and by experimenting but its only when you have to debug a real product that it crashing that in my experience the ideas finally set in place. Then you really get it.
As I'm sure you're aware multithreading/parallelism is the hardest thing in software to get good at.

One trick I've used is to represent the items of data in a particular problem with physical objects, apples or oranges are good. Discuss the problem with your colleagues but each person can only speak when they are holding a piece of fruit and once you've assigned each piece of fruit to a part of the problem they can only speak about that part of the problem when they're holding that piece of fruit.
They can put their peice of fruit down on the table at any time and pick up any number of pieces of fruit one at a time but only from the table.
It's a fun game to play and pretty soon you work out how to lock and unlock your data effectively and to make best use of your resources.
Sometimes it's best to have a boss who tells people what fruit to take or even hands it out and takes it back. Sometimes it's best to work cooperatively by following simple rules to avoid deadlocks when you need multiple resources.