Pack the heaviest items at the bottom of the suitcase (orientation = standing upright). This usually means your shoes.
Use Packing Cubes - primarily to contain like-items. They come in a variety of sizes and can be purchased individually or in sets. The various sizes are ideal for different TYPES of clothes (not quantity). T-shirts. Sport shirts / dress shirts. Slacks/Jeans. Underwear. Socks. Stack the cubes flat against the handle tubes. If you use Slim Packing Cubes, they go great between the handle tubes to create a flatter surface.
Some people like to roll their clothes, others like to fold. Experiment and see which is right for you.
The true beauty of packing cubes is when you get to your destination. It's easy to quickly and NEATLY find the various articles of clothing you need without rummaging through your bag. You'll be able to re-pack in minutes.
If you are really pressed for space:
Roll up socks and stuff them inside your 2nd pair of shoes (do you really need the 2nd pair?).
Bring one belt that reverses from black to brown.
Keep your wardrobe simple. Fewer dependent Top/Pant color combo's.
Place belts flat-packed around the inside perimeter of the suitcase. They take up no space when packed this way.
If your suitcase has an outside compartment/pocket, pack this with whatever garment you will want quick access to while traveling (think sweatshirt for the cold airplane). Remember to pack this item first, then account for the thickness of the front section (when closing/zipping the bag) when packing the main compartment. The physics of cubic volume are pretty straight-forward. If the main compartment is full, and the lid is bulging, you're asking for trouble (see #12 below).
If you are carrying your bag on the plane, put your TSA-approved volume of liquids in a 1-quart zip-lock bag and place it in an outside pocket. Check out TSA.gov for details (double check this please).
If you are checking your bag, here are some toiletry kit tips:
If you absolutely MUST pack your own shampoo and conditioner, squeeze out as much air as possible before sealing the bottle, then tape the tops of your bottles closed. You'd be surprised how often they open and leak. Place each container in their own zip-lock bag. This helps reduce altitude expansion and contains leakage.
Pack your toiletry kit at the TOP of the suitcase (just beneath the top grab-handle) preventing the weight of the contents in your bag from crushing things.
Pack a new toothbrush each time you travel. Few people replace them as often as they should and who wants to use a toothbrush that's been packed away from your last trip?
Buy the specially designed giant zip-lock bags that have the one-way valves. Not the ones that require a vacuum hose, the ones that allow you to ROLL the excess air OUT of the bag. Pack your dirty clothes into these things and squeeze/roll them as flat as you can. Not only are you saving space (who cares if your dirty clothes are wrinkled), but you're also keeping the "dirt" where it belongs - away from your clean clothes. If this is too much for you, take advantage of the hotel laundry bags.
Toss a dryer sheet in your suitcase. Yes - it works… your clothes smell fresh - especially if you don't follow step #9.
Depending on how often you travel, it's a good idea to spray Fabreeze inside your bag to freshen it up.
An overstuffed bag that explodes on the luggage carousel is NOT the fault of the zipper - it's your fault for over packing. Zippers do not have magical powers. If it takes two people to zip the bag closed, you're playing with fire. Use some common sense here… and take some clothing OUT of the bag, or get a bigger bag.
Put a copy of your flight itinerary and hotel reservation INSIDE your suitcase. Countless bags are lost each year - even though they begin their journey with a routing tag. This simple inexpensive trick will save you the hassle of loosing a bag forever. It'll also help a lot in increasing your chances of the airline getting it to you while you're still travelling.
Travel Electronics take up a lot of space and can add a lot of weight. If possible, carry your USB cables to charge your gadgets, instead of the bulky AC adapters. But be sure your device will charge via the USB cable. If you're travelling internationally, be sure that any AC adapter you bring is 100/240 volt compatible (read the tiny print on the adapters - it'll say what voltage it accepts).