Market Trading is generally regarded as the most efficient way to make gigantic piles of ISK in Eve. It is efficient because it is scalable - if you can make 10% on a 1 million ISK investment, you can make 10% on a 1 billion ISK investment with little additional trouble. Compared to something like mission running or incursions, where your income is effectively capped at say 120M/account/hour with full attention, trading is capped only by your available capital and your commitment.
I have three accounts; each account has one primary character and two alts. All of my alts are trading alts; that is they have a minimal set of skills to place and modify orders and contracts. The set I use as a minimum is:
- Trade IV
- Marketing III
- Procurement III
- Accounting III
- Broker Relations III
- Daytrading III
- Social I
- Contracting I
I put each alt in a different region, in a station that is either a hub of interest (e.g. Jita 4-4) or central to the region to allow reaching all systems with the remote skills at III. Of course, training these skills higher is handy, especially for alts that deal in a lot of orders.
As well as developing my own trade routes, I can contract items for sale (exploration loot etc) either at their station of residence or using courier contracts. The trade alts can then babysit the sell orders freeing up the mains for pew pew. Courier contracts are also great for Fedexing new ships to my mains - my Jita alt often fills ship orders for my mains and other people in the corp, delivered by space truckers usually within 24 hours.
I’ll go into more detail as I learn, but I’d really recommend anyone getting into trading to check out Eve Mentat, which is a very good trade tool under constant development. Also you can catch the developer, Thart, on Google+.