I've tried a few sets of replacement mirrors, some were even worse than the factory ones.
The only ones that really worked were so wide that they looked ridiculous on this small scoot. Luckily those cheap things fell apart within a few months, solving the dilemma of looks vs. use.
Now I'm back with the (decent quality) stock mirrors and I just got used to them.
Just like codger78 said : it's just a matter of finding the right angles and getting to know the limitations.
In my case a mall spot (wide angle) mirror does help though, the only difference with codger's setup is that i've stuck it to the outside edge of the left mirror so it "looks" around my shoulder so I can see the cars coming up right behind my back...
Never trust mirrors alone, always look over your shoulder before turning or changing lanes, this goes for driving any vehicle by the way...