We have all (most of us, anyway) practiced the first 12 minutes of the game against the computer enough times to make it second nature. There are many players who can do that very well. However, for many, that's where the proficiency ends. There are many pieces of advice for how to proceed from here, but putting sometimes contradictory snippets into a coherent game plan is easier said than done. I want to focus on just one aspect of the midgame, villager control.
One part of villager control is seeing the big picture. What resources are required for your overall strategy? And more importantly, in what ratio? This is really the toughest part of villager control, as you have to mentally add up all the resources you need for your game plan. This is a simple linear function, but is made more complex due to the varying collection rates through the ages. Take for instance a minor scout/slinger rush on a land map. 900F(18 peons), 500F(tool), 125F(armor), 100F/50S(stone mining), 480W(2 granary, 2 SP), 525W(3 barracks, 1 stable) = 1625F/1005W/50S. If you want 3 scouts and 9 slingers, you need an additional 660F/90S, for a total of 2285F/1005W/140S. Unless you got really jerked around by the random gods, berries are 90%+ efficient, so you need 5100 Villager-Seconds(VS) for food and 1800 VS for wood, i.e., 75% of your villagers' time should have been on food, 25% on Wood. This is a little simplistic, because it does not assume that you want to do things like wall, tower, scout or get the woodcutting upgrade, but it gives a basic framework. As you get the massive offsets of the ages out of the way, villager assignment becomes much easier. Farming is 100% efficient, and woodcutting 80%, so to supply your needs of constant CA production from 4 archeries (6 CA/min) requires 10 farmers and 9 woodcutters, and 1 additional woodcutter for farms. As Staffa pointed out in his post some time ago, resources sitting in the bank represent wasted villager-seconds. Try to balance your production with your needs.
Efficiently convert your only real resource, villager-seconds, to the resources you need. Common wisdom tells you that if you have a gold-dependent civ, get villagers on gold early, maybe even in Stone. Big mistake. No civ needs gold prior to Bronze, so wait until you have completed the Gold Mining upgrade. I believe neilkaz pointed out that 3 villagers with gold mining beat 4 without, so use those villager seconds to your advantage. If you have average luck, you should have been able to do a wood-gold pit by mid-tool, and can pull all the miners you need as soon as the Gold Mining upgrade is complete. Minimize your walking. It's just wasted time. Stone Mining is nice, too, but unless you have a guardian angel, you need to be walling or towering as soon as you hit Tool, so you have to live with some inefficiency here. Woodchoppers should Everyone seems to have mastered queueing up villagers as soon as Bronze hits, but don't seem to know what to do from there. I can't tell you how many times I have come into an enemy's town, or viewed an ally's town with writing, and seen 3 or more peons standing around the TC. Don't make peons unless you have a plan for them! You have got to get them working, or you have just wasted food. Get used to using the middle mouse button, and listen for those Villager Complete sounds. That goes for boats, too. I have won games I should have lost because someone didn't clear a path for his fishing ships to get to his docks. Another fine point is Beware of shifting too many villagers too quickly. Ideally, you should be able to shift them one or two at a time. I have won too many games in which I was outclassed because enemy peons were running from task to task, wasting valuable villager-seconds that were needed for materials for defense. The only exception is needing many gold miners once you get the Gold Mining upgrade. This will have to be done en masse. Lastly, send builders with your army. 4-6 should be sufficient. They should be one step behind your army. I usually make them build 4+ military buildings, 4 towers and one TC near the front at an abandoned gold mine. Generate peons and military at this outpost and strip the gold. Once the miners are done, move them to either some close trees or follow the army. If you have to retreat, the enemy has the choice of destroying the buildings or leaving an outpost between his van and his town. Without STs, this will buy you a minute or two, a good deal for 800W/600S. Comments? Questions? Critiques? Keep your stick on the ice.