Compress and Compact
Most of the time, you can make more room in your bag without leaving anything behind: Just squeeze. Pick up a few compression sacks. They're usually worth the price, since they make it easy to pack clothes, especially underwear and socks, into a much smaller space. Nobody cares if your dirty laundry gets wrinkled, so a compression sack is the perfect place to stow it later in your trip.
A compressible option for your nicer pants, skirts and shirts is the packing folder. It saves space with the added bonus of cutting down on wrinkles.
Once everything's in a compact package, don't forget to use your suitcase's built-in pockets, straps and other compartments to organize your stuff and to take advantage of unused space.