Acquire the habit. That's all, really: but you must understand that the habit means: write all the time; think things through in terms of how you you would write them up all the time. Which is to say: become a writing nerd. It's an uncomfortable truth, in an age in which 'nerdiness' is so denigrated, but there you go. The nerd live for, eats sleeps and breathes his/her chosen object of nerdiness. That's what you must become.
Put another way: I read an article about Jack White in which various beautiful women who had dated him explained the main reason why they'd broken it off ... they'd started seeing him because they thought they'd be dating a rock star; but it turned out they were actually dating a massive rock music nerd. And in a nutshell that's the crucial difference. Many people aspire to become writers because in their head they're in love with the idea of being a sort of literary rock star. But to be a good writer, you must make yourself into a massive writing nerd. That's the price you pay.