I think that one of the best things that you can do to improve your musical skill is to play with a fellow musician. I mean by this is that if you find yourself a person who plays your instrument by yourself and never wants to open up and play with another, I think you are missing out on a lot.
By playing my guitar with others I realized that I picked up so much from their different methods of learning. Everyone develops different methods of teaching themselves whether you are self-taught or you are taking lessons.You may also find easier ways to do something. For example if you are willing to learn different tuning methods or just something you don't no how to do you can just watch what they do or you can even just ask. You are also able to compare different techniques and methods of learning when you start to play your instrument with someone else.
Also by playing with another I think that you become more motivated to learn something new and take on new challenges. For example if you are just a strict metal person maybe the person you are playing with likes blues. Maybe by playing with this person you can open up to different styles of music. Another good thing about playing with another is picking up songs that they learned or maybe you would like to share with this person something you have written.
When you play your instrument with another you may also feel the motivation to take on bigger challenges. Such as you may have found yourself before to just look at the tablature for a song an tell yourself that it is to hard and give up completely. When you play with a friend or whomever you may see that person is able to play a more difficult song and may motivate you to try something at a new level.
I think that by playing with another musician that you will find yourself taking on greater challenges. By comparing your methods and techniques you are able to learn so much about your instrument and maybe some quick and easy methods. I know that by doing this. It will help you to become a better musician.