Out of all the gil making strategies, tradeskills take the most time. However, it is well worth the payoff. Although you can choose any profession you want, maximizing your gil output can be helped by below tips:

Choose 1 gathering and 2 crafting professions.
Not only will this allow you to maximize profits, it will also make sure that there will be materials available for each crafting professions, courtesy of the gathering profession.

Leveling them all the way up immediately is a bad idea.
Switch every time you gain five levels in a job. Not only does this allow you to sell off the items you made, it will also keep you from getting bored.

Spend a lot of gil by buying materials.
Some materials can only be acquired from other professions or by item drops from monsters. Spend 10,000 gil to buy materials, as soon as you reach level 25, in crafting items for your trade profession.

Do some levequests.
Not only are levequests useful for gathering, but also for crafting. Although it doesn't use many materials for making items, it will also allow you to earn some gil and experience. Turn in only normal items though, and not high quality ones. To earn experience and seals, run grand company quests, although they won't earn you gil.

Make high quality items.
No matter what level you are on, high quality items will always sell. Therefore, strive to make high quality items. This way, you can earn some of your money back, with extras to boot.