For the early-ish game, one of the best one-two combos a mage can do is Grease-Fireball, since the first spell dramatically boosts the power of the second and a lot of companions have Fire-based attacks that synergize with it as well. It wanes in effectiveness when fighting enemies with multiple actions per round (you have to be more careful with the timing) or which strongly resist Fire damage, but it's pretty good for the early game and is still useful later.
Once you start getting access to Tomes (or have enough money to do training for new Powers, which will most likely happen first) a good suggestion is to pick up various abilities you didn't choose on leveling up, and possibly some White Mage/Charmer ones as well if you want to add some other damage types or buffs to your bag of tricks. Don't be afraid to swap Powers out between fights depending on the area, because Black Mages can cover a huge variety of damage types (but not all at once) so you can become much more effective if you're willing to swap skills around instead of looking for one build to stick with for the entire game. And if you use some cross-class skills you can cover even more ground.
If you plan to make use of summons, make sure to boost Presence either when you level up or by drinking the appropriate buffing liquor, because that's the stat that determines how powerful they are. Willpower is the stat you definitely want to focus on boosting because that's what almost everything you do will scale off of. And yes, as mentioned look for gear that improves Spellpower, Spell Penetration or both.
