"The pen is mightier than the sword" -- Terry Pratchett
The idea that you can accomplish more with well written words than you can through physical violence is an old adage. It is still true though. If you don't believe me then just think of some of the most interesting bits in human history and ask yourself why you believe certain people were good and others bad. It all comes from the viewpoint of the ones that wrote it.
This tool can be extremely powerful. Examples can be found if you look at wartime propaganda... no matter what side they are from, the written word moved many. Without a doubt, the most powerful and influential writing can be found in the Bible. But we aren't going to get into that.
My point is that the pen can be powerful, but just as any other tool it must be properly, regularly, and carefully maintained. When writing with a pencil you must sharpen it after some time. It isn't just the physical writing tool that must be sharp however. The most important tool used in writing is the mind.
It is in your mind that thoughts, words, paragraphs, ideas themselves are formed. I believe that the more quality writing and variety of writing that you consume -- read -- the easier it will be to demonstrate stronger skills yourself. Think of it as an apprenticeship to the masters of literature... Whomever the masters of your favorite genre happen to be.
Another major aspect is that you must practice. Therefore, you must write! It doesn't have to be a lot, it doesn't have to be all the time, but if you don't use your skills and work at fine tuning them, it will show in your end product. There are many clichés and adages for this, but you probably already know them.
You must put your passion into your writing. You are creating something. Whether you publish it, share it, or just keep it to yourself, you will find that your writing is better when you are at least interested in what you are writing. How can you ask anyone else to be interested if you aren't?
This said, you also have to expect criticism. I actually look for it. Well, not the mean spirited criticism (which you may get at times), but the constructive kind. I know that my writing can improve... everyone's writing can improve. It is difficult when you have put a lot of passion into something so much that you can tell if a character would like a movie or a song because you have invested that much in building them. It is difficult, but necessary. Many times we are too close to see smaller errors, to realize that we didn't explain something clearly enough in the writing because we already know in our minds, or even to see how certain events are completely missing.
When you are invested in your work, it is hard to look at it objectively. What I have found to work the best for me is to set my work aside for a time, and then pull it out and read it as if it is the first time. If you have the imagination to create characters and settings then you have the imagination needed to pretend you are reading someone else's book... at least to some extent.
Other times you need assistance. you need someone impartial to evaluate your work and honestly point out inconsistencies or errors. While what they say will be tainted by their own personal preference and style, you can usually glean something from it.
Don't forget that if you want to publish or if you are writing material for school, work, or whatever you will have an audience. Your writing has to reflect and be accessible and understandable by the readers. Granted I am new to the publishing world, but I am here. I have done a lot of reading and writing in my life. I know that not everyone will like what I have to say, but I hope that some will. I enjoy writing and I want others to enjoy reading my work.
Ultimately, keeping your pen sharp will be your job. It isn't always easy, but if you enjoy something you don't give up with the first hints of difficulty... you push yourself harder.