And lastly, do lots of quests.
I can't speak for other towns, but in Bastok:
Ring Bearer:
Talk with Nabu latteh (Think that's the name, it's a mithra) infront of a bridge across the street from a traders house. She'll give you a fire crystal and ask you to deliver a copper ring to her daughter in Bastok Mines. Go to the jewelery shop right across the street and pick up a copper ring for 75 gil, head to Bastok Mines. From the entrance turn left and go down the ramp, follow the wall to the right till you come to the last door infront of a dead end. Give the child mithra the ring and she'll give you a letter. Return it to the person who gave you the quest (Without reading it!) and she'll give you 200 gil, for 125 gil profit. Can repeat this about 5 times.
Mommy's Helper:
On the same road as the Ring Bearer quest, head all the way east and go to the last traders house. There's a mother who forgot to do grocery shopping. Her daughter tells you what items she wants at what time of day. This can be done over and over again.
Wild Onions: After 12:00 until ??? for 350 Gil, dropped off Goblins
SleepShrooms: ??? until 6:00 No clue

Treant Bulbs: After 6:00 until 12:00 for 200 Gil, dropped off Sapplings.
Flint Stones:
In the house next to the one with Mommy's Helper is a lady who complains about her husband and son. Every now and then (haven't figured out the timer for that yet), she'll run out of flint stones. Give her 4 for 100 gil. This one is repeatable too.
Those are the only quests I know of for making easy money (Hope I get the strat guide soon and that it has some others)
Aside from that, be sure to get all your armor and stuff from the auction house, it's usualy waaay cheaper (I bought a level 10 helmet for 400 Gil there, it sells for over 1200 in the armor shop). Also whenever you have an item or a stack of items go to the auction house and check out the price history on it. It might be more effecient to sell it there than to a npc vendor.
Copper Ore's sell to NPCs for 2 gil, but 50 gil at the auction houses and 12 Crystals sell for 168 gil but usualy 1000+ at auction houses.