Why does my early 2011 MBP hibernate instead of sleep?
First, try disabling hibernation (safe sleep) entirely:

sudo pmset -a hibernatemode 0

Then, to remove any trace of other previous hibernations, run:

rm /private/var/vm/sleepimage to free ~4 GB (in my iMac's case)

Should you ever want to turn Safe Sleep back on, run
sudo pmset -a hibernatemode 3

If you want to manage all of this with a simple System Preferences pane and it's worth four dollars for you, there's always SmartSleep.