Some good tips in here, but I'd like to add my own:
Know your ship and ID.
This one should be obvious. Knowing what ID and ship to use can be a hrad task. Remember to stay within the Tech Chart and to know the consequences of your ID.
Having an Independent Trader ID is all well and good, but you might get some funny looks if you go to Malta to pick up Cardamine. Likewise a Smuggler ID can get lawfuls asking uncomfortable questions even if you aren't breaking any laws.
With ships, know what house the ship was built in. Flying a Shire in Kusari isn't a good idea, and neither is flying a Behemoth in Liberty. Also, don't pick a pirate transport/train if you plan to do some legit trading in House space. You'll get asked to leave at the very least. The best ship for trading (nearly) everywhere is a Borderworlds Transport; anyone can use it (including Indie Traders and the like) and it can hold its own in a fight. It's expensive at first, but worth it once you can afford it.