Many of the most profitable materials are the lower level ones (you level too fast to come up with a huge amount of materials which are in high demand; at higher DoL levels everyone ends up with piles and piles of the stuff and have to sell at 5 gil a piece). Gather supplies you can turn into other supplies (IE Latex > Rubber instead of Iron Bars > Frypan) to grind and then sell the extra results. Also, Walnut logs and Iron ore. Get some.

Once you get around 20-25 you'll definitely want to start using leves. Again, plan ahead for what leves you want to crank out and gather those supplies. Triple turn-in leves (require three items to turn in) can be repeated without the use of additional leves up to twice. 9X the materials for 3X the rewards, great if you're getting the most out of each leve and have the materials. Leve rewards of 45-60 shards are also great and can help you avoid having to go back and grind out shards (or buy them).