I find the Multiply mode the best way to put stuff behind or into other things. The best thing to put inside another is something with a white background if you use the Multiply mode method.
Looked for a glass bowl and found this one:
http://www.hottuna.co.za/props/vases/shared/Glear%20glass%20bowl%20vase.jpgAdjusted some stuff (Auto Levels, Auto Contrast, and Auto Color).
Then wanted to put a mug of beer in there:
http://www.discoverahobby.com/beer.jpgPasted that sucker on top of the glass bowl image, set the Mode to Multiply and added a Layer Adjustment thing to push up the Saturation a little (+54) and then decided to set the opacity of the beer layer to 80%. That made this sucker (not the best thing in the world, but it certainly looks like it's inside the bowl, eh?):

What would make this even more believable is adjusting the shadows the new beer glass would make, but alas, I guess I'll skip that for now.