There isn’t an easy answer, but as Chris Messina said…you just have to keep at it. Getting better at writing takes practice. Try writing different styles. Try writing if you were someone else. Experiment and write, get the words out even if they seem like crap, it’s part of getting into a groove.
Most of the time I don’t hit my groove in a piece until I’m about 150–200 words in. Even when I have a great idea, I need to warm up. I need to flex my muscles to help the ideas form.
The last thing, the thing that every great writer I know does, is read. A lot. The less I read—and a diverse range of things—the crappier my writing becomes. I’m too stuck in my own head to write cool things. Reading other inspiring, or just cool, stuff always helps to get things going.
My two cents.