1) play such that you aren't losing vast numbers of guys. Obviously this isn't always possible but playing conservatively on a long campaign makes a huge difference. Example: charge calvary, let them have a few attacks, then pull back and charge again. Do this in waves and your horses will last way longer.
2) Try to use standard troop types. If you have 8 units of all the same type of spearmen and you lose 10% of each unit you can merge units and top up 7 of them and send the 8th weakened unit to be retrained. This is especially advantageous if your weakened unit is more experienced, since when you retrain it they'll all be veterans. Contrast this to a full stack army with 12 different troop types. You can't merge them, so significant losses mean you have to retrain big chunks of troops at once.
3) Have a stack of reinforcements. I often have a full stack with the general and then trailing a few squares behind I put another 6-8 units of extra troops. These are garrison, wounded units, or other specialty units that I don't think I'll need in my regular army right away.
4) Use mercenaries for the really dangerous fighting. Send mercs through walls first, use them to meet frontal charges, and do as much as you can to get your money's worth. Since you can't retrain mercs and new mercs don't respawn until the unit is dead or disbanded, all but the most valuable mercs (cretans, rhodians, balearics and elephants) should be used as fodder in place of your regular army.
5) Try to take big cities when you need to retrain. This isn't really intuitive since you usually need a full strength stack to take a big city, but big cities are more likely to have the buildings/population you need to retrain. Thus when on campaign try and make your last action with an army before they rest the taking of a large city. This doesn't always work, but it certainly can. Like if you're playing Thrace against Macedon, hit Byzalora first, then take Thessalonica and retrain there. Caralis -> Carthage or Thapsus -> Carthage works the same way. Hope this helps.