Practice seems an obvious answer but it’s an incomplete one. What’s needed is practice backed up with better reading. You need to be influenced by great writers while you flesh out your own sentences. No one desiring to draw a new automobile design would look to a Pinto, a Maverick, or a Yugo. Instead you would take cues from Lamborghini, Ferrari, and Porsche. Average influences will only produce average results, and penning five hundred or a thousand words a day will only get you more of the same. More average.
Melville for cadence, Orwell for honesty, and Hunter Thompson for outrageousness, are just some writers that are bound to positively affect you. Reading poetry, especially extended nonsense poetry, gives you an ear as to what words and sentences sing and those which merely hum. Consider the opening of Lewis Carroll’s Jabberwocky (external link):
‘Twas brillig, and the slithy toves
Did gyre and gimble in the wabe:
All mimsy were the borogoves,
And the mome raths outgrabe.
Here, it’s not the meaning that’s important, it’s the pattern and the sound that the words possess. Such utter originality cannot be borne out of simply pushing pen against the page, day after day. While Carroll was a genius, he had his influences. We can be influenced, too, chiefly by fine, classic writers. We need to read and read well. Without great guidance and inspiration we will simply create a Pinto, a Maverick, or a Yugo in print.
Update: February 7, 2015
This is a link to a speech Bob Dylan recently gave. (external link) In it he discusses what influenced him; his marvelous and mysterious songs did not appear by themselves. Someone described the speech as masterclass. I totally agree.