Perhaps you need to learn a bit more of a wheelie technique to help lift that wheel? Or just not pulling up properly? I like instructional videos like this one.
http://westcoaststyle.net/mountainbiking.php

But for immediate gratification …..

I just think of lifting my front wheel as if I were actually going over an obstacle
http://www.mountainzone.com/videos/p...o.asp?vid=7623

Over even just bunnyhopping with no obstacle.
http://www.5min.com/Video/How-to-Bun...our-Bike-10142

More extreme is an actual manual or wheelie
http://ibikeride.com/Mountain-Bike-S...elie-drop.html

The big thing is learning each bike’s behavior. Some bikes are easier to bunnyhop evenly. It was easier to lift the front wheel on the hard and softtail bikes I’ve had. I have to think a bit more about keeping my front wheel higher on my FS bikes which means I should probably go practice more.