I don't do much of the following because they don't sit right with me, but:
  • Don't gather mats yourself unless the only listings on the mb are overpriced - You'll make more crafting than you would gathering, even with the cost of buying mats factored in
  • Undercut by 1 gil - The average player doesn't know about taxes and city states and will buy the lowest priced item. Undercutting by larger amounts can see you move stock faster, but it can tank the price by too much
  • Buy items from npc merchants and put them up for sale on the mb at many times what you paid - You'll get players that will pay the extra gil for the convenience of not having to teleport around the world, as well as players that don't know npc merchants sell the items for much less. Fashion Report items are a good example
  • Buy items when they are cheap on the mb and relist at prices many times higher. Shortly before a new patch, materia will be bought up and then relisted once the patch drops
  • Run alts - Allows you to have more/all specialists at your disposal, more inventory space and more listings
  • An item's value is what players are willing to pay for it - Glam weapons that cost very little to make went for several million because the buyers were unaware of what went into making them

That's for crafting. If you are skilled with combat, selling runs will get you as much or more than crafting, depending on skill and some other factors.