Farm easy monsters in newbie areas, when coming back by Chocobo. Level up low-level subjobs, getting lots of crystals and items.
Don't sell very cheap items 1 at a time, wait until you stack 12 (example, Yagudo Feather), and then sell.
Jeuno has a 200-500 gil surcharge on AH selling, but often the prices there are much higher (take advantage of those high-levellers!).
12 Beehive Chips is 3000+ gil.
12 Fire Crystals is 1000+ gil.
12 Wind Crystals is 1000+ gil.
12 Lighting Crystals is 1000+ gil.
Most goblin armor pieces are 500+ gil each.
1 Silk Thread is 600+ gil.
12 Yagudo Feathers are 1000+ gil in Jeuno.
The Ambrosius lost mail quest is 500 gil per 4 pieces of mail, and these are easy to get (just kill some Rarabs in Sarutabaruta and Tahrongi). Creepy Crawlies is 600 gil per 3 Calculi. Other random repeatable quests can net you lots of fame AND a consistent influx of 200 gil gifts.
I may not be an expert on how to make money, but I do have very reasonable armor and weapons at MNK/23/WAR/11, and about 35,000 gil to boot. (I know that's not a huge amount of money, but it pays the bills.) So it's really not that difficult, and, actually, breaks up the monotony of levelling all the time.
yurik (Unicorn) MNK/23 WAR/11 -- Unicorn