Some great free psd files + some good inspiration can be found on designmoo
And it was mentioned earlier, but deserves another – GuideGuide is one of the most useful photoshop extensions I have ever used, sooo worth the free download 
Armin wasn’t a big fan of it either over on brand new. Pretty thorough write up if you want to know more 

Awesome! Congrats + very deserved, one of the true originals in this marketplace 
Yay Envato, congrats! proud to be one of em 
...but if you’re creating for the most wanted, you should have it at 326ppi:
Retina Ready User Interfaces suitable for Apple iOS. These are full User Interface layouts or collections that are in 326ppi format. You may include a 72ppi version too, but we are specifically requesting 326ppi format graphical interfaces.
@EladChai Distort works really well, too
You could use the box that’s similar on a layer behind, and start adjusting the panels on top of it using distort, then just remove the layer when finished..works well for me. Don’t forget to add a little realism with shadows and highlights once it’s put together, too. Good luck!
Posting an image and linking to the vid is the only way I’ve seen it done here. Don’t think GR allows you to embed..
Another great resource is The Dieline ..it’s the #1 packaging blog online, so you should be able to find just about anything there 
MarkBrodhuber said
Hey guys, Just wanted to quickly pop in here to let everyone know that we’re going to be adjusting the prices in the packaging category to try and compensate for the extra work involved with creating for the category. And depending on the category’s results we may need to adjust the prices a bit more.
Thanks Mark! Great news!
