If you want to write--make it an everyday date.
If you want to write well--prepare to spend time every day doing it.
Now, maybe you're saying: "But I have classes to attend/a job/lots of demands on my time! How am I supposed to find the time to write at all? Can't I do it on my day off? Or during Spring Break? How about once I've gotten people off my back?
Well, yes, you can write on days off. Or on vacation. Or after the latest deadline's been met.
If I may be so indelicate, however, let me ask: how much writing can you really do if you're putting it off for more "opportune" moments?
And what do you do when something comes up on that day off?
Authors write for hours at a time--some three hours a day, others eight or more.
Relax. I'm not telling you to rent an office, dedicate 40+ hours a week to writing, or even carve out three to four hours a day for it.
Not that these aren't good goals. Your goal is simple: keep a small notebook (or smartphone) on hand. Jot down what you're thinking about, what you see--whatever catches your attention. I recommend starting with a 15-minute session, and sticking to it, every day.
Once you've got the hang of it down, increase your writing time: 30 minutes, an hour, two hours, and so on. If you can't write for an uninterrupted period of time, write at small intervals, until you've accrued your daily time.
And if it takes a while to make the habit stick--don't be too hard on yourself. Just keep writing.