I'd agree the best way to learn defensive stuff is 1 on 1 or by flying as observer with a good stick (gee, guess that rules me out

)...but here is a little general programming for your brain.
The biggest mistakes most folks make when on the defense are as follows ( just my .02 guy's) comments welcomed.
1) Moving to early
2) Using to much E in move
3) Evasives which serve no tactical purpose
4) consecutive moves that cancel out
5) Moves that actually play to bogies
strength
We all see guys (and/or are guilty of these)..Guys start scissors when your 1.2 back and you hose em as they go back and forth...Barrel roll and roll out 400 in front of you

...325 mph pony pulls 6 g break turn as you barrel in your nikki and cant accelerate away from your hi yoyo.
To me the starting point is Identifying your Goals...
1) stay alive...these are the hard evasives that are a sign you've screwed the pooch earlier..or been bounced etc. These occur with the enemy in or close to gun solution
2) Minimizing moves...these occur in close contact but are to avoid allowing bogie to gain a gun solution.
3) Equalization moves...the purpose here is to level the playing lield
An example...
Bogie comes screaming in from hi 6...barrel roll with full throttle chop to evade..back to wep after (example of 1 above)..he blows thru and goes up to initiate 2nd pass...little slower though now...this time start scissors..he goes for one realizes he's to fast and goes for Hi yoyo to keep some E (move 2)...Split-s at break hoping to extend far enough to reengage (move 3) Obviously lot of hypotheticals here but ideally your worst case is he's where he started with less E and in a perfect world your now co-e and dealing with a simple angles problem.
IMHO what seperates the wolfs from us sheep is the ability to intuitivly string these moves togeather in an efficient manner.