I don't know where you are or what your doing/not doing, so forgive me if some of this is basic.
First, you know you can hot key your groups, right? And you can hot-key your buildings? And then you can set your rally point on your hero from your buildings. So what this means is say you are out exploring a map with a group of footmen and your hero. You just hit 1 to grab them all, and walk. While walking, you can hit, say Zero (it's what I always hotkey my barracks with) and then tell your barracks to make more footmen without having to move your screen away from the action. Then the footmen will come out and walk straight to your hero, and you can just quickly add them to your hotkey group on 1 with Shift+1. I think this is very helpful.
Second, make sure you learn some of the basic hotkey commands. Including, for instance, that Holy Light is the T button. Don't forget that your first hero is always that F1 button, second hero is F2, and so on. That's something that I forgot early on.
Third, in the campaign (which I've only played through once for each ROC and FT on normal), a mix of units is probably a good idea. But if you're ever playing the computer or a person, the general strategy is to mass the same units to the point where they don't have a counter for that many of that particular unit. Even a boatload of ghouls with a Demon Lord and his Vampiric Aura can be a very effective strategy. But in the campaign, you'd have to tell me more about the individual board that you're on. What race, what's the challenge, etc.
Fourth, Magic Users. This is harder and you just have to get used to using the hot key commands and a quick mouse (which no doubtedly you have). One's micro skills can make the difference in a game. If it's an army against some small-time creeps, remember that you can walk away whatever unit that's taking a beat-down and the creeps will usually switch to another unit to attack. Just cycle through your units so that none of them die but all of them are weakened, and you can take out creeps more effectively. You can hold down the "alt" button so that all hit-point bars are showing on screen and then you know which ones are about to die, and then run them away.
As far as your particular resource management issues, I just suggest the hotkey-ing the buildings and simply getting used to checking back in with your buildings. But if you have the hot key pattern down, it goes by really quick. Like, I just hit Zero, then H twice and all of a sudden I have two Huntresses queued up. It took like 0.5 seconds.
Soooo, if this doesn't help, can you give me more details about what's going on in your campaign? What Race? What hero do you have? And what spells do you have? What magic users are you allowed at this point? You are playing Frozen Throne, right? Or are you playing Reign of Chaos?