the key thing to remember in a yak is that you are constantly being underestimated. it isnt the most common plane, and most people's knowledge of the russian planes ends at the la-7 or la-5.
yak 9u has good top speed, good climb, and it is an above average turner. you wont turn on the level of a hurri or spit, but from my experience you are probably on the turning level of a poorly flown niki. the yak is not a good slow speed turner, however, so if you see your opponent getting angles, dont get into a stall fight. use your acceleration to extend away.
the yak is a tough target to hit. keep this in mind if you have a bogey d400 on your six but you are extending- a couple of jinks and you will be difficult to hit, dont get suckered into a turn fight when it is not necessary.
when i am in a yak, it really seems like most of my kills come from an enemy underestimating the yak's capabilities. this is usually something like an overshooter doesnt increase angle of climb enough on extension and i can match his climb for a bit, or a pony trying to turn fight.
as always, maintain situational awareness.