Maximum mana can be increased by use of passives (Spiritual Attunement is the only one I can think of off-hand), or items (mojos, voodoo masks, and ceremonial knives can all roll +maximum mana, and possibly some legendary items).
However, increasing maximum mana is not the most effective way to avoid running out during a fight.
It is generally more effective to increase your mana regeneration rate, reduce the mana cost of your spells, use one of the methods of gaining mana from killing enemies, or, better yet, combining them.
Much like +max mana, +mana regen can be found on mojos, voodoo masks, and ceremonial knives. In addition, the Zunimassa's set bonuses can give you additional mana regen. Even when spamming spells with a high mana cost (like Bears), mana regen helps a lot.
You can also get a small increase in mana regen from the templar follower, or much larger temporary boosts from empowerment shrines and the Big Bad Voodoo skill.
The Soul Harvest and Spirit Walk runes that return mana, are not terribly effective, and frequently there are better choices. Rush of Essence can be useful if you are focusing on the skills it affects in your build.
The bonus from Spiritual Attunement also increases your mana regen by a small amount (more so if you also have +max mana items).
Blood Ritual, on the other hand, reduces the cost of all of your spells by 15%. This can be quite significant, particularly for the big-hitter spells. You can also find items that reduce the casting cost of certain spells by a flat amount.
Finally, there are some ways of triggering additional mana gain that can be quite effective. The Zunimassa's top set bonus also gives mana for each enemy killed, in addition to the base regen rate.
The passive Gruesome Feast gives you 10% of your total mana every time you pick up a health globe, in addition to a major, but short-lived, buff to your intelligence. There are several ways to increase the number of globes that you can get, so this can be a fairly useful passive.
Grave Injustice gives you 2% of your life and mana for each enemy killed in your pickup range, in addition to lowering cooldowns. This can add up very quickly if you have a high pickup radius and are killing very quickly.
In general, combining some or all of these is the best way of increasing your ability to continually cast. However, be warned that at very high MP levels, mana regen that is related to killing enemies becomes very unreliable, simply because you aren't killing that fast.