I'm not sure how much practice you've gotten in since posting this, but there are a couple things I'll mention. Slipping the clutch is a bad idea. You're going to wear out your clutch super fast if you're slipping it to get the front wheel up in the air. Also, becuase of clutch wear, you would need to increase the frequency of your oil changes.
Clutching the front wheel up is done by pulling the clutch in and popping it back out almost instantaneously while under light to moderate acceleration. This is more aggressive than your slipping, but it's not going to overheat and wear out your clutch plates as quick.
I'm unsure of why you want to pull it up in second gear when you're doing it in first already. Option one being you want to go faster, and option two being you want to carry it farther. IF you're looking to carry it farther, then don't do it in second. Keep going with the first gear wheelies and bring the front wheel up higher. Bringing the front of the bike higher creates more wind resistance, and puts more of your weight back allowing you to shut of the throttle slightly to keep your speed down.
If you just want to go faster, well, that's what second will give you. The technique for getting it up should be the same for all gears if you're using the clutch, just slightly more aggressive. And COVER THAT REAR BRAKE.
