I'd imagine if you have Viru and Gambhir on song, or someone like Yuvi or Symonds out there batting for like 10 overs, then you could use the powerplay on them.
Or a pinch hitter. Runrate slowing, take the powerplay and the pinch hitter. If the bowling side is forced to use a few part-timers, then you could pull the powerplay out and whack away. Maybe from the 35th over onwards, to give some momentum coming into the final ten.
But I don't like it. It would be more exciting if they were decided on random. Like after every five overs a ball is pulled out of one of those lottery machines, with 'Powerplay' or 'No-Powerplay' written on it. Then there could be a siren blaze around the field, and the umpires dance around for a bit before the inning resumes. And in the mid-innings break we could have a massive dance party, and have balloons and streamers and cheerleaders. And maybe we could rename it from Cricket to Ultra Uber Hi Fi Supersonic Crazy Fun Bat and Ball Game, and the players could be Ball Whackers and Ball Throwers, with 'keepers being called SuperFly Alien Men with Gloves. And there could be car races around the track, and clowns doing tricks every time a four or six is hit (Which could now be called Lots of Runs and WAYYY More Runs!), and there could be a Disco in the pavillions, with celebrities doing the commentating while plugging their new movies. And ofcourse, by now the ICC is run by the BCCI, so cricket is now called The Tata-Sky/Indicom/Birla/Reliance Mega Hyper Fun Sporting Event, and no other countries play apart from India, India A, India B, India Has-Beens, India Superstars, Pakistani Legends, Bangladesh superheroes, and a Rest of The World team picked full of players who have nothing else but nice things to say about India...
Wait, where were we?