Thanks for all the replies,
Your suggestions seem to confirm my own opinion about how I was going to handle this.
#1. Multiple encounters are necessary if the players are going to learn the lesson about not using all their resources at once. [Thus making the one encounter per day more challenging].
#2. Making house rules about regaining powers seems like a fix, but I'm loathe to go fiddlin' with a game system that may very easily become [more] unbalanced because of it.
#3. Minions make good meat shields, sure...but they really fall easily to area/blast attacks that do even the smallest amount of damage. In my case the problem is the spell Thunderwave. It's 3x3 effect, and he has a (predictably) large attack bonus (well, if you were a wizard, wouldn't you put points in intelligence). Plus it's an at-will spell, so he can use it whenever he wants.
#4 Terrain and environment does matter. In one recent dungeon encounter I managed to neuter the mage somewhat by placing the monsters on a 30ft ledge and giving them crossbows. My minions could retreat backwards and come forwards to attack, had defensive bonuses. Sure, it became a siege effectively, but it was an interesting one-off encounter. The players resolved it by agreeing to a ceasefire, whilst their opponents withdraw from the dungeon. Then, promptly breaking the ceasefire they filled the leader full of crossbow bolts. They enjoyed it.
Let me give you some specifics now. Maybe that would prompt some more ideas.
I have (5 or) 6 players. There is a fair spread of classes, wizard, cleric, rogue, paladin, ranger and (occasionally) fighter.
They are level 3. Wizard has the daily spell Sleep (5x5 area), and Encounter spells Icy Terrain (3x3) and Colour Spray (5x5). The Cleric routinely casts Shield of Faith (11x11 centred on cleric) to give the players +2 AC at the start of the encounter.
What I've got in my mind already, for an encounter that is important to the ongoing campaign, is this.
To the north-west is a region occupied by goblin tribes. One of these tribes has been contacted by the major antagonist of the campaign seeking lackeys. They have refused to submit to the will of the bad guy, who has blighted the crops and animals of the tribe as punishment. Winter is fast approaching and so the tribe has sent out warriors to steal food and animals from the human lands to the south-east. As well as raiding outlying farmsteads they are ambushing travellers on the road. That's the background.
The PC's are escorting a group of 20 pilgrims (old men, women, children and babies) to a shrine to the north. They are passing through the region currently being raided by these goblins. My plan is to ambush the group at a river crossing, hopefully splitting the PC's. The objective of the goblins is not to kill the PC's or pilgrims, but to steal their food, mounts, etc... I see them using some of the pilgrims as "shields" against the PC's. The paladin "picked up" a ring that translates goblin that I planted in an earlier adventure. He'll be able to communicate with the goblins. I want the PC's to have a tough decision to make (especially with regards to protecting their pilgrims - they refused to rescue some farmers, letting them die, in our last game session after realising that they couldn't deal with the ghouls attacking the farm if they split up the party, and they refused to leave the pilgrims defenceless). If they choose to fight, I want it to be a tough fight that either ends with them being captured, or the capture of some of the goblins (I can fudge this by having a pilgrim knock one or two of them down). Either way, there has to be some communication after the combat where the goblins will let drop the clue that will players will immediately pick up [having encountered this blight elsewhere]. What they do after that develops the campaign story (and given my players, is completely unpredictable so I shan't even try to guess).
Can you fantastic people come up with more suggestions to make this a really memorable encounter? I'm looking for ideas about how the goblins might use terrain and environment (temperate, edge of civilized human lands, autumn.) and tactics that might be used to avoid the problems mentioned in my initial post. If you can think of a believable chaff encounter that'd be great.