First of all, I'd DEFINATELY charge banshees when I get the chance, because if you don't THEY'LL get the +1 attack for charging instead of you. Beyond that, the +1 strength for charging means you'd wound on 2+, so it's worth it if you can get a slight edge in numbers on the banshees.
Also, a very simple and easy bit of help is buying a 75 Whirlwind. Hell, buy TWO if you have to. The ability to rain an ordinance template down on those large guardian squads no matter where they go will DEFINATELY edge the favor back to you.
Also, having bike squads in reserve to counter charge are a GREAT idea. Their toughness is so high that it makes nearly all Eldar foot trooper units worthless against them.