I use a wide, flat-bottomed plastic food container, approximately 10"x10"x2" tall. If I am fortunate enough to fill it 2/3rds full, I grab another and start filling it. It's also important not to let your berries sit in a container too long if there is any reasonable stacking of the fruit because the bottom fruit will slowly succumb to the pressure from the fruit above. I wash, allow to air dry on an absorbent material and freeze what I'm not using within a couple of days as soon as I get them inside the house.

I like a bucket (no bigger than a gallon, so the berries don't get squished) with a handle so I can tie it around my waist so I have both hands free to pick. If you don't have one, just cut the top off of a milk jug (keep the handle, of course!). Plastic bags aren't a good option, because of the squish factor and because they really hold heat.

Two-handed picking is far faster, although many home growers do not pick large enough quantities to make that method a must. Two-handed picking also helps in finding more "hidden" berries that would otherwise be found as moldy or over-ripe at the next harvest.

Assuming we're discussing caneberries, any container depth of more than about 3 inches will start juicing the bottom layer of berries, but that is fine if the berries will be processed into jam or whatever. Rinsing pre-smashed berries for fresh eating is difficult.

Any berries destined for fresh eating as single berries should be picked in the morning; they will be firmer and not be damaged as easily as on a hot afternoon.

I'm not sure how you would keep a berry-harvest bag from being stained!

When I harvested berries as a youngster for local growers, we always used a metal pail with a handle and looped it through our belt or had a carabiner-type device to attach it to pants/belt. These pails would hold enough berries to fill a 12-pound crate (12 hallocks), so the pail must have been about 2 gallons. Those berries went straight to the cannery. The grower would send everyone home early if the afternoon was hot.

Raspberries

Save-On sells 10" pies in plastic containers. They have a black bottom with a snap on lid. I use the bottom if I am giving them away and the top (slightly bigger) if I am processing them myself. They hold 6-8 cups of berries.

Saskatoons, currants etc.

4 litre ice cream pail. The berries don't bounce out. :)

I have found the lightweight three gallon plastic ice cream buckets just perfect for berry picking. You can sling it over one wrist and pick with both hands. I always save those instead of recycling them because you can stack one inside the other to take with you picking, they can go through a dishwasher and not stain. I used them for all sorts of chores, keeping the food ones separate from the 'dirty' ones I use for household chores.

Thanks!