Depends a bit on what you're facing. I fly the A8 almost exclusively but the a5 is basically an A8 on steroids, so some general rule can be applied.
Rule bumer one: stay fast! Keep speed around 300 or more at all times; zoom up only when you know you're clear. In the A8, this comes almost automatically but tha A5 is a decent turner for a FW so it's tempting to turn just that littl bit extra.
You'll outroll all your opponents ad medium speeds.As speed increases, you have to watch out for the American b&z'ersas they'll easily follow you.
If in trouble, take the fight low-ish. 3k is a good dogfighting alt for the A5 - it accelerates well here and as a good climb rate. Try to keep a few k' - not so mucn for running but as a buffer that'll enable you to build speed and with it a possibility of escape.
If caught low and slow, use the 190's weaknesses to your advantage - snap rolls are highly effective. Beware though; I've augered more than a few times doing it at alts around 100 feet

.
Effective gun range is around 325 - 350 tops. Aim for deflection shots where you are presented with a big target - the A5 is not a follow around and get a dead 6 shot plane.
Sometimes, surprise your enemy by turning into him. Only do it if you have enough alt to be able to build speed back up again. It's useful when you have wingies and want to buy some time.
Climb when you can, but set default climb speed relatively high, and climb only when you're clear of danger. Keeping speed around 300 is far more effective than giving it up for a few hundred feet of altitude. Excpet to be on the defensive against t&b'ers like the Spit a lot - they'll immelman right onto your butt and try to stay glued. Keeping speed up will allow you to rope them and regain the offensive.
The 190 ain't a good runner, but it is a powerful diver. It has excellent trimming characteristics and good elevator authority. If pursued by 109's, a last ditch move that works is a speedy dive to the ground followed by a violent high g crank upwards. The 109 might auger (seen this a lot of times).
NIK's are best dealt with by keeping speed around 300 and easily outrolling them. Getting slower vs these planes is very dangerous.
P-51's and F4-U's are dangerous - but if you're low, your acceleration and climb rate will give you enough to either hold your own or reverse the situation.
If you're attacking these planes, beware that they can outturn you, outgun you, are faster, have better low speed characteristics and are uglier. Speed up, deflection shots, zooms and keep it going.
That's about it for now

------------------
Baron Claus "StSanta" Von Ribbentroppen
9./JG 54 "Grünherz"

"I am the light at the end of your sorry little tunnel." - A. Eldricht