Ring of Essence: Creating heroes WITHOUT losing your sov's essence using Imbue Champion
It was mentioned to me in a reply and a lot of other posters that you don't have to use your Sov's "Imbue Champion" (which permanently costs your Sov 3 essence points -- reducing his/her max mana) ability to make heroes into casters. All you need is to find the Ring of Essence, which grants a +1 Essence to the wearer. I actually can't remember where it is, so someone who has that memorized ... I think its a 2nd or 3rd level Quest or perhaps a goodie hut.
Anyhow, with this one ring, you can make every hero you have into a magecaster if you do it carefully (some might consider it an exploit). Once you remove the ring, you no longer get the essence, but if you have a non-casting hero bear the ring, take the hero into battle and level up, you can then increase the hero's essence; the hero is then permanently a caster. You then remove the ring from that hero and trade it to another hero.
To really maximize this, I generally take a full stack of zero-essence heroes with my sov (with "organized" trait so everyone in his stack moves at his speed and buy every movement buff I can -- mount, cloak, ring, etc.) and a few summoned monsters (whatever the strongest summoned monsters are available; as I get access to more or the current ones get damaged, I replace them using my new casters if necessary). I select my next ring bearer based on who is closest (in terms of fewest XP needed) to levelling up -- use mouse over to check over the meter. Don't rely on the meter itself, since if your heroes are on different levels, a lower-level hero may level up before a higher-level hero even if the higher-level hero's meter shows it being closer to the next level. Of course, if its close (or you are going into a huge battle where everyone will probably level up), its better to favor the higher-level hero since it'll take longer for them to reach the next level. Once they've levelled up and you've put the level-up point into their essence, transfer the ring to the next hero closest to levelling up and repeat. Your sov and heroes can generally stay safely behind, use a couple summoned monsters to do the actual fighting ... be very careful when facing archers, though, that's when you need to consider using ranged spellcasting lest they snipe some of your heroes before your summoned army can reach them.
Remember a stack can only have 12 units. When your casters in the stack have enough mana to teleport (only need one to teleport a stack, really), consider doing so to make room in the stack for new heroes you recruit to join up and gain essence. Don't forget to keep recruiting heroes; if you have too many zero-essence heroes to put them all in your Sov's stack, send them to a city so your Sov can later shed the new casters, teleport to the zero-essence heroes stack and go back to making new casters. If you use Organized and movement buffs on your Sov to keep your stack moving quickly (which lets you battle monsters faster), keep your Sov with the group; also get sigh improvements on your Sov; this will really help you find and hunt down monsters. As you go along, if your summoned monsters take damage or stronger monsters become available, take them out of the stack and have them go guard a city where they'll also heal up; you can replace them using one of your new casters if your Sov already has summoned monsters.