For years I used the 3 gallon Zip-locs to pack in and still do to a lesser degree since I have started using Eagle Creek's compression packing cubes. LOVE them! I am a meticulous packer because I would prefer to spend the time packing and not have to iron once I arrive at my destination.
I have never had a zip-loc or compression cube opened and/or rifled through a luggage inspection (you know, when you have that TSA paper in your checked luggage saying they inspected your bag?). I've had a suitcase soaked after a tremendous storm hit MCO while the baggage crew was loading the plane (there was lightning so they of course abandoned the bags out in the downpour), but my clothes were safely packed in those huge zip-locs and packing cubes. The packing cubes aren't waterproof, just water resistant like a windbreaker would be.
Here's how I order my compression packing cubes….. I'm signed up for emails from my favorite travel companies like TravelSmith and eBags. When they have a 20% off everything, (here's the great part! you ready, fellow travelers??), I then log onto my chosen airline's website,
www.aa.com, then log onto Aadvantage Shopping Mall and see if my favorite travel company has any mileage specials for ordering. Almost always there is a deal for multiple miles per dollar spent, usually around 4 miles per dollar, but it can be less or much more than that! I order through Aadvantage Shopping so I get extra mileage credit AND a special sales price! WOOHOO!!!
With the 3 gallon zip-locs, I carefully fold my clothing items, stack them and carefully slide them into a ziploc, then I close the zipper about halfway, sit on the bag (yes, I said I sit on it! LOL), and finish zipping it up so that I end up with a very compressed bag. Keeps things from slipping around and keeps them nice and clean AND takes up less room!!
Similar technique with the compression cubes: carefully fold and stack into the cube, zip it closed, then zip the compression zipper. One tip: keep one finger pressing down ahead of the zipper so as not to get the soft nylon fabric stuck in the zipper. 'Cause that is NO fun to try and get out!