While some travelers go for the 'throw it all in and hope' packing method, most prefer a little more organization in their suitcase or backpack. Internal compartments can be useful for separating clean clothes from dirty, but that's about as far as it goes.
Luggage space is often at a premium as well, especially when traveling to colder climates or with a mixture of 'work' and 'play' clothes. While some luggage comes with compression straps on the inside, outside, or both, they don't reduce space all that much. Typically, they're just used as a last-ditch attempt to get an overstuffed bag to close.
Fortunately, there are inexpensive ways to deal with each of these problems, as well as some innovative options that sort out both at once.
Available in all manner of colors and sizes, packing cubes are made from a semi-soft, lightweight fabric. They have a zipped, often meshed lid, and are designed to keep various items together in your luggage. For example, you might keep shirts or blouses in one cube, underwear, and socks in another, and miscellaneous items like books and chargers in another.
Being square or rectangular, they're ideal in suitcases. They're not always as useful inside a backpack, but depending on its shape, it can still work relatively well.
The main aim of packing cubes is to keep your luggage organized, letting you quickly lay your hands on an item without having to dump everything else on the floor to find it. Opting for different colors and/or sizes for each cube is a useful identification technique, or resort to masking tape and a marker pen.
What packing cubes won't do, however, is give you extra space in your bag. In fact, unless you have square luggage and use up all of the available space in each cube, you'll often end up with less usable room.
Compression sacks generally resemble a heavy-duty plastic bag, with a zipper for loading and unloading, and a one-way valve for squeezing the air out. The idea is to fold bulky items like sweatshirts and jackets into the compression sack, then vacuum-seal the bag to end up with a flatter, smaller – and often waterproof -- package.
Like packing cubes, compression sacks come in different sizes and colors. Their effectiveness depends largely on what you're trying to compress. Solid items like books won't compress at all, while thick t-shirts, sweaters, and the like will squash down to as little as a quarter of their original bulk.
They can also be used to keep things organized, although they aren't as convenient as packing cubes if you need something in a hurry.
Don't forget: while compression sacks can provide plenty of extra luggage space, they don't reduce the weight. That's important if you're trying to avoid checking your bag or carrying it up a few flights of stairs. Despite typically being made of heavy-duty PVC, a compressed sack doesn't have much give, and won't deal with sharp objects well.
A few companies have come up with innovative ways of combining packing cubes with some form of compression.
The HoboRoll, for example, is a five-part packing cylinder with compression straps to cinch the contents, which can also be carried by itself for an overnight trip. The company also offers a smaller, ultra-light version for when space is super-tight.
I've used different versions of the HoboRoll for years, on everything from week-long trips with a tight carry-on allowance, to a month-long walk across Spain with a 30-liter backpack. It's great for freeing up extra space in your bag and fits better in a backpack than a standard square packing cube.
Eagle Creek and others also make what they call compression cubes, which work like a standard packing cube but have a 'compression zip' to help squash everything down. You can also buy larger (20-30 liter) compression bags that operate much like day packs, with flip-top lids and straps down the side.
None of these varieties, however, reduce space as much as a dedicated compression sack.
There's also the simple option of putting compression sacks inside packing cubes – it provides maximum flexibility, at the cost of extra packing time.
So Which Should You Buy?
If you don't have a tendency to over-pack and just want to keep things organized, opt for packing cubes. They're light, relatively cheap, and mean you won't be the person everyone is waiting for as you try to find that vital something at the bottom of your bag.
For those who just never have enough luggage space, compression sacks are a better choice. They require more work to unpack and (especially) pack, but can't be beaten when your priority is to fit as much stuff as possible in a limited space. Waterproofing is a useful side benefit.
When you really need flexibility, check out the combination options. They don't squash down as much as a dedicated unit, but even a bit of extra space might be all that's needed to get that stubborn zipper to close.