Tactics
In a pre-gunpowder world, cavalry form one of the most dangerous parts of an army. If used correctly, they can prove vital in securing victory. However, do not be fooled into thinking that they are equally effective against every other kind of unit. Heavy cavalry can repel light cavalry. Archers can pick cavalry units off before they get the chance to charge. Infantry can set their longspears to receive a charge, causing you to receive a sudden surplus of equine shish kebabs.
Attack the Flanks
In warfare, positioning is crucial. Each position has its advantages and disadvantages. Examine where your enemies are before committing expensive and easily exhausted forces.Infantry normally use staff weapons: spears, pikes and so on. These make frontal attack difficult, so make sure you assault the infantry's weak points: the flanks and rear. Remember Lance Corporal Jones's lesson: They don't like it up 'em
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Consider the Terrain
The cavalry charge is devastating, but only really works if your horsemen are able to keep formation and hit the enemy in a solid wave. Doing this depends on three things: uniformity of equipment, to ensure no horse is significantly more encumbered than its fellows; discipline of the riders; and evenness of ground. As a rule of thumb, only charge over flat, clear ground. Make sure you know the area first. Know the location of all natural hazards: bogs, quicksand, ankheg nests, portals to the demiplane of violent buggery, lava pits, that sort of thing.
More importantly, make sure the enemy hasn't seeded the ground with caltrops unless you want to see hundreds of armoured horses tapdance in unison.
Don't Exhaust Yourself
Cavalry in general and heavy cavalry in particular have only limited stamina. Don't send your horses in at full gallop right from the start. Choose your moment carefully. Leave the business of harassing routed soldiers to units that have sufficient support and aren't lugging fifty pounds of ironmongery on their backs.