Beyond the main hubs there are a multitude of secondary hubs of varying size. Most regions have several fairly large secondary hubs and many smaller hubs.
What isn't immediately obvious is there are clusters of secondary hubs associated with the main hubs. Several large secondary hubs, spread across multiple regions, all within a few jumps of each other and with one of the main hubs more or less central to the cluster.
For example there's what I think of as the "Minmatar Cluster" covering the regions of Metropolis, Heimatar, Molden Heath and Derelik.
Teonusude (Molden Heath)
This cluster is rather unique because not only does it contain 2 main trade hubs (the others only have 1 main hub) but it also encompasses regions in both Minmatar (3 regions) and Amarr space (1 region). In the other 3 trade clusters (1 for each of the other 3 main hubs) the regions are all of one faction.
Molden Heath is a small region with Teonusude being by far the largest of the small handful of secondary hubs there and one of the few high sec systems in the region. There's an active PvP community in the region which drives a small but thriving market that's surprisingly good for such a small region.
Derelik is a large region in Amarr space that's very close to Rens. Seven of the other secondary hubs in Derelik are larger than Tanoo, but I like Tanoo because it's closer to Rens than any of the larger hubs, directly straddles the route to Rens and is a bit of a chokepoint between large parts of the region.
Travelling from Rens (the central hub of the cluster) the distances involved are:
Rens to Hek 6 jumps
Rens to Teon 5 jumps
Rens to Tanoo 4 jumps
All distances are using high sec jumps only (no low/null shortcuts if there are any).
As a trader you can simply send big loads to Rens or Hek and supply the rest of the cluster with a series of short trips from there.
I generally courier a big load from one of the other main hubs to Hek and post anything that has good prices there. Then I load the rest up plus any excess stock that was sitting on Hek into a blockade runner (gotta love that unscannable cargo hold!) and fly to Rens. I contract everything to the Rens trader, he posts everything worth posting there, then contracts everything back to the Hek Trader (including any excess stock that was just sitting on Rens).
Rinse and repeat for Teon and Tanoo. Anything that was gathered up along the way and not posted anywhere goes back to Hek. Some of the gathered up stuff gets posted on Hek, if there's at least a billion or so worth left over that I don't think I'll be able to sell soon in the cluster, it all gets shipped out (usually via courier) to one of the other main hubs.
I tend to do this about once a week (a different cluster each time) or about once a month for each of the 4 clusters I work.