My first tip is, unless you are seriously committed to becoming a better writer, is to give up now. Without 100% committment, you will be bound for disappointment. And even if you are 100% committed, you are still probably bound for disappointment.
My second tip, and listen carefully - www.critiquecircle.com
I'm only familiar with a handful of writer improvement websites, but this one is by far the most comprehensive. It has, and continues to be my most powerful tool in improving as a writer and I would be nowhere near the writer I am today without the help of the people who frequent this website.
It starts by you critiquing others. Then, you submit your work and let others critique it. The more people you critique, the more who critique you in return. It's also free!
In fact, for me it's a work process. I get my critiques in, gain enough to post my work each week, and get other people's critiques in return.
It's also got forums where you can ask questions, discuss character development, plot, other aspects of writing, or just vent about being rejected by yet another agent.