Often it depends of your oppenent, and the level you're playing. Some levels you can get the ball up on top and bouncing happily with just a few power bars, which should leave you with enough to charge a special.
If I'm playing Mini-T, I try to keep the specials lined up, so my oppenent doesn't have any time to rest. FYI, if you charge multiple Blow specials in a row, they will get sent one after another - there is no wastage.
With Ducky it's a different story. I prefer to get a large store of power bars, and then release as many ducks as I can (preferably more than 5). This results in a "duck jam" right in the center, near the bottom of the screen. Then I follow up with ducks as I get the power bars. If the game lasts long enough, and if you have enough spare power bars, this will make you oppenent's screen one big duck pond, and they will fall, yes, yes they will. fear the duck.
For Bombastic, use 'em whenever you have a good number of power bars (halfway up is nice). Charge specials in succession, so that your oppenent gets flooded with bombs. This strategy worked much better in earlier betas, but Bombastic seems to have been neutered a bit since then.
Ioni is really only effective (against me anyway) in batches of two or three - I can still function with only one batch of flowers, but more then that and I'm flying blind. Which isn't actually that hard, since you can bounce the ball straight up and down the left and right sides pretty easily. Just make sure to give the ball a good whack, other wise it'll go to slow, and then you will fall, yes, yes you will.
Well, that's all for now folks, since this has kind of turned in to an essay on the pops special powers!
-janski