You practice writing. You practice directing. You practice camera work and editing. But, you're forgetting an often overlooked area that is absolutely essential to successful films: the paperwork.
Another area to pay attention to, is your paper trail. Keep track of all your project expenditures, and SAVE THOSE RECEIPTS! For one, if you fill out tax forms as an independent contractor, those receipts can be written off as business expenses. More importantly, keeping track of how much you spend is the only way to know if you're project is on budget. With low budget projects, make sure to account for donated expenses like labor, equipment rental, and other line items you got a deal on. It's one thing to say you made a great film for $500. It's another thing to say you made a great film for $500 that should have cost $5,000. See how that plays into making a good impression? It will also give you an honest look as to why films are so expensive to produce. You won't always be able to get labor and equipment for free. Keeping tabs on your spending now will allow you to know what kind of budget you need in order to accomplish bigger projects later into your career.