Related to How do I separate 2 drinking glasses that are stuck?, but also quite different. Sometimes I have some plastic five gallon buckets stacked up (usually 2 or 3, sometimes 4) for storage, and they slowly sink together. I might not need them for years. Then when I do need them, I can get them apart, but at some expense:
If I poke a nail-hole in the bottom of one, it will release the vacuum, and the bucket will come away. Not good if you intend to use the bucket for liquids, like I usually do.
I can get the air compressor, plug it in, wait for it to pump up, find a hose and fittings, put them on, jam the end between the buckets, and blast in some air. They pop apart nicely. However, the air pump isn't always in working order, and sometimes I have this problem in places where the folks don't have a compressor.
Holding the buckets between your knees firmly, and twisting the top one. Doesn't work on tight buckets. You can do this for 1/2 hour sometimes to no avail. Same with the method of rolling the buckets, squashing them a little, rolling them, squashing them again, etc. It works unless the buckets are real tight.
Getting a narrow steel rod and jamming it between the buckets to let in some air. It works also, but I avoid it as it deforms the buckets.
I'm not sure if there are any conventional methods, because this isn't really supposed to happen. Now you know my problem, and the methods I've tried, and why they weren't satisfactory. What is an efficient way to get multiple five gallon buckets un-stuck that doesn't involve taking a long time (20 mins or longer is a long time), and doesn't involve deforming/defacing the buckets?