Buying paint in larger quantities...

Not that I know of. If you'll find a way to buy it in larger quantities, let me know. :) I'm currently using Vallejo acrylic paint in bottles with droppers, so I can reliably drop predetermined number of drops without throwing anything away. Also, i've become an expert in deciding, how much paint I'll need for the job; rarely I throw away more than a couple of drops. :)

At least I can airbrush as much finish as I want, since I've bought 2 bottles of future... :)

Vallejo also makes a line "Air" which is prethinned for airbrushing, perhaps that will help you out?

I agree that you shouldn't keep thinned enamels or lacquers. But I keep my thinned Tamiya Acrylics. I just make sure never to put thinned paint back into the original bottle. 

Thinned (with distilled water) humbrol acrylics are going bad after couple of weeks; they still can be used after a serious stiring, but I have a feeling they lay down worse. I haven't experimented with thinned Vallejo acrylics; no need. Vallejo Air are pre-thinned, although not as rich in pigment as Vallejo Game Color. (Not that they are not rich - they are, but Vallejo Game Color are extremely bright and pigmented).