Do you find though that, due to prices changing, you have to go through it all again on a regular basis?
It seems very hard to do, and I'm not sure if the systems/planets I'm discounting are right...I'm not sure if I should look at exporting from one system type to another, or whether it's only supply and demand (in numbers) that matters.
well, i run this route every four weeks or so, and i have to change 1 or 2 systems then.
my strategy to set up a trade route would be:
a) make a list of high value commodities. that are commodities with 800+ Cr galactic average prize. why? because 20 percent profit on a 1000 cr commodity are 200 cr, while 20% profit on 400 Cr commodity are 80 cr/t...
b) note down, which economy produces which of those high value goods, also note down, which economy needs those high value goods (you can find both infos in the commodity description in the market). you will end up with a list of 20-30 commodities. those are in 90% of all cases the only ones, which are worth trading.
c) start somewhere by buying all those commodities from the market which are available and below galactic average, have a look at supply (if low, check traffic report), and go to the next system, where you know the economy should need your commodities. note down profit of all goods your brought, sell, but hold your samples, and buy all high value goods from the market as sample, go to next system... etc. some people use special ships for scouting trade routes with very good jumpranges, like an DBE or Asp. for me scouting is part of the game .... it takes longer, but i make little profits anyway.
do you want to set up a route in special region of space? it can be a good idea to have a "home base" (high tech economy is always a good idea, because those produce many high value goods...) - then you simply take your samples and check all systems you can reach with one jump fully laden... you will have a trading net of a dozend stations soon.
then comes the fine tuning. how far is the station from entry point? if you trade for profit, you don't want to spend 10 minutes in supercruise. how much profit can you do on the way back? etc. have in mind that you want to have a good average profit. if do a-b-a trading, making 1200 cr/t on the first way, but only 100 CR on the way back, isn't as good as making 800 Cr profit on both ways.
from experience i can tell you, that this can be very suprising and annoying - you find a dirt cheap supply of beryllium, and no system nearby, where to you could sell them with profit AND buy something below galactic average. or you check out 8 dictatorship to sell your battle weapons...
a classical trade cycle will follow most of the times these economy types: HI/EX - EX/HI - REF/IN. or, a full cycle: ref - ind - ag - ex - hi - ref