I second (or third - I lost count!) the eBags packing cubes. I travel frequently for work and these have been a Godsend. I learned a new trick on my most recent trip - I found those "squeeze the air out" plastic bags on sale at Marshalls so I bought a set. Wouldn't you know, the packing cubes fit perfectly inside the plastic bags and when you squeeze the air out, they get thinner by about half their size!!!
Best wishes for whatever you decide to buy!