Hmmmm... I never seem to have a lack of mana, if anything I almost think there is an overabundance.
I general grab attunement for my SOV, Doubles mana production initially. Get shard harvesting early in order to start building a base of production.
I almost never have any persistent enchantments running including imbue champion. I ALWAYS cast before battle and then dispel afterwards. Also I generally only use spells on big battles, e.g. when I'm about to capture an enemy capital.
Currently sitting on turn 138 of a game. Hard difficulty. I have 738 mana in reserve. However in this game i did get lucky and score two shards near my capital.