What I did :
A Big Ship : Rank is tied to profits, profits are tied to ship size ( all else being equal ). Get the biggest ship you can and run missions when available supplementing with trading to the destination, pure trade when not. A Cutter will be your best choice, followed by an Anaconda, then a T-9. At least until the rumored Panther comes out.
Local Rep : Good missions require local rep, so keep that in mind. A regular run/route helps. I ran the TUN circuit and it worked well, gained Fed rank as well. Just watch for stupid runs to stations 200k LS out..
The Cargo : Reliable commodities is also key. Prog-Cells and Performance Enhancers ALWAYS sell at a good profit, somewhere. Slave trading is highly profitable, but morally reprehensible. Trading Basic Meds to outbreak systems is highly profitable, and even better when you have a return run, but it changes weekly so you need to stay on top of it. Also, moving large amounts of specific items tends to depress the local need for said item. Route Fatigue is a real problem with bigger ships.
CG's do really well for profit margin, but it changes weekly as well. Not sure if the bonus at the end applies to your rank ?
One thing I have tended to notice ( but not done any deep analysis on ) is that a station tends to be a specific percentage point above/below galactic average ( GE ). That is : If they are a 'selling' station, all items tend to be the same percentage below GE. 10% of a 300 credit item nets you 30 credits per ton. A 10k credit item nets you 1k per ton. The same tends to be true of 'buying' stations. They are either + or - an given percentage. If you're going to be hauling junk around the universe, don't haul cheap stuff. "It takes money to make money" rings true in ED.