Cav tactics for me are often quite simplistic: stick with the infantry!!!
Really, i can't stress this enough. I know it's really cool and fun to ride with a cav formation of 10 guys and charge the enemies... but it's usually a bad idea.
Instead, try this: get the whole team to defend a position (hilltop, village, etc). Get your ranged guys into good positions, get te footmen to defend the ranged and the cav to defend the footmen. Then wait for the enemy infantry to engage your team's footmen and only then charge them from the side or from behind.
Cav in small numbers (less then 10%) is much more effective supporting the melee instead of going off on some charge.
I can't even count the number of times my team lost, even though the ranged guys had discipline and set up a good position, offering a nice area of 'friendly ranged coverage'... shielders and other footmen that patiently wait and protect the ranged guys, thus making sure they get to do what they are good (shooting enemies and denying an area for them) without being rushed by some infantry or cavalry... only to see our cavalry rush out and first chase the enemy cav all the way to their main force and see them get dismounted due to ranged enemy fire, pikes and enemy cav.
I guess the point i'm trying to make is this: unlike the hollywood-style cavalry charges we've all seen in cartoons and movies, cavalry in cRPG is mainly a
support class. We cav are great at quickly and efficiently killing unaware or distracted enemy players.
The one way to guarantee they are distracted is to wait untill they are a) within range of your team's archers, throwers and crossbowmen and b) are engaging your team's infantry and locked into melee fights.
Be careful not to run over too many friendly players when doing this. But with a bit of fancy maneuvering, you'll do fine and you'll rack up 5+ important kills every round.
Good cav is patient cav. Good cav supports melee.
In the above scenario i've assumed a map where your team is being defensive. When your team attacks or charges, the same principles apply. Wait until they are engaging your infantry and attack them from the side or from behind.
The one big difference when attacking is that you'll want to make sure their attention is divided between your team's footmen and you. So you can be slightly more agressive. Still, harassing the enemy team as cav doens't have to involve you charging into them. Instead, be a threat by getting behind them but don't get into range until the moment they are fighting your team's footmen.
The biggest and hardest lesson for cav, i think, is restraint. Taking less risks and staying alive. Any cav that's alive in the 'endgame' of a round is often worth it's weight in gold, if you get my meaning.
A last thought: cav formations are not useless at all! But try to use those cav formations to charge in at the right time (when they are engaging your team / when in range of your archers etc).