There are a few common places.
1) On the firewall above the pinchweld in the center. Just need to move the wiring and brake line a little. This may be the easiest place to put it, not the best looking or functioning. (gets hot there, may reduce life of unit) This is where mine is for now.
2)If you have a CAI than you can put it down on the frame rail where the air box was.
3) Inside the fender on the drivers side. Need to remove the splash apron to install it, but may be one of the cleanest installs.
4)Not common... but works. Summit sells a mount that you can use. this places it on the passanger side inner fender, bellow/beside the MAF meter.