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It's all about the Ziploc
Ziplocs have to be the traveller's best friend.

Need to squish all your socks and undies into a compressible bag? Ziploc.

Need a container for your shampoo, lotion or contact solution? Ziploc.

Need a sleeve to keep your journal nice and dry and less battered, and keep the ticket stubs and mini maps and pressed flowers and pen from falling out? Ziploc.

Plastic protector for your airline tickets so they don't get sweated to death in your waist belt? Ziploc.

I usually bring 4-quart size freezer Ziplocs and 3 gallon size freezer Ziplocs in addition to anything I've already packed in a Ziploc. They are so handy for sorting and waterproofing, and keeping stench-ridden clothing out of your clean stuff.

If you're super-cheap like me, Ziplocs are great for packing a lunch, saving money on food. On my last trip, I took an entire box of sandwich bags and pretty much used them all up for sunscreen, snacks, vitamins, you name it.

With all the fun airport security, the personnel can search through your stuff much easier. Not as much unzipping of things.  It also helps when your looking through your bag, it doesn't get as torn up when you can see what's in the bags.

I discovered them for my last trip.  They especially worked out when my tiny conditioner cracked open, luckily inside of a Ziploc!

I'd also recommend taking plastic grocery bags along, since they're considerably cheaper than Ziploc bags and have handles.  I prefer wrapping the stinky clothes in free plastic grocery bags than Ziplocs.  It's not a bad idea to bring one or two of those from home for larger quantities as laundry or mucky shoe bags as plastic grocery bags in Europe are often NOT FREE! If you're going to want a bigger plastic bag, don't count on getting one in Europe for free, particularly in Germany. They charge you at the register.  Usually the ones you have to pay for in Europe a sturdier and better quality, but I wasn't talking about those. I meant those cheap thin ones that puncture easily and are rather disposable. 

Husband thought I went a little overboard when washing underwear in them. Hey, he had clean underwear, didn't he?  Put a little detergent in the Ziploc, add some warm water, swish it around, then add the clothing, zip, and knead the closed bag. Works pretty well. Some times sinks don't have plugs, and some sinks are not so clean, so I think this was the reason this strange practice was invented. I fill an empty film canister with liquid detergent, and keep it in the zipped Ziploc that I use for the cleaning, just in case of a leak.