Translate only into your native language.
As a native English speaker, I stick to Korean-into-English work. It's better for me and better for my clients, who often have no way of settling any battle of opinions between native-Korean translators and their editors.
Never do quick-and-dirty work.
Some clients request a "draft-quality" translation that "doesn't have to be perfect" because it's for internal use only. Yeah. That sounds like something that would come back and bite me. I don't do that.
Strive for quality over quantity.
Although this principle may seem obvious, it's sometimes hard to remember in the day-to-day grind of a freelancer's life. Tight deadlines and clients with expectations based on movies (you know, the ones where the Egyptologist goes back in time four thousand years, somehow speaks ancient Egyptian perfectly, and translates the hieroglyphics in real time for his time-traveling companions) can really test a translator's resolution to stick to his principles. I am determined to do quality work, even if it means missing some the-client-needs-it-yesterday jobs. Unlike our fictional Egyptologist, I don't do time travel.
Base all translation on solid research.
I study background for all projects, even ones that involve subjects with which I'm very familiar. Everything changes; technology, culture, and language evolve at the pace of fashion. Trying to keep up can be a wild ride, and it's not always possible. Project-by-project research helps fill in the gaps and ensures that I'm using up-to-date language and terminology.
Complete all the work yourself.
Farming out work to other translators is fine, I guess, as long as the client is okay with it. I haven't had much luck with that approach, however. All translations are my own, and I take responsibility for them.
Preserve your own intellectual property.
By this I mean that my translation methods and practices belong to me, not to any client or translation agency. I provide clients with a finished product, but not any job-related research notes, vocabulary lists, or CAT segments.
This is a life saver. Trying to work as a freelance translator while coddling a bad procrastination habit could kill you. Literally. I enjoy my life and would like to live a bit longer, so I do my best to avoid slipping into a nasty, heart-attack-inducing habit of putting off until tomorrow that first draft I could do today.