1. Roll up your socks and underwear and stick them inside your shoes.
Doing this will free up a tremendous amount of room in your bag. 

2. Speaking of shoes no matter how long your traveling try and stick to bringing only two pairs. One for sightseeing, working out and casual food spots and one for more upper scale activities like hitting the disco or upper scale restaurants. 

3. Rubber bands. Use rubber bands to tie up your clothing and create more space in your bag. ( Not recommended of course for high-end garments As an added benefit this will keep your clothing neat and organized in your bag. 

4. Be conscious of your color palette when selecting clothing to pack. If your color palettes are cross compatible, you will have more outfits to wear but less clothing to pack. 

5. Versatile, low-maintenance clothing is essential. Fabrics like Merino wool keep you warm and cool depending on the day's weather and can we worn for days without fear of odor arising. A couple other things to consider try and pack clothes that hid stains well and fold up nice and small.