First things first: lay out everything you expect to take, including your suitcase. If you've been on a vacation of the same duration before, remember what you didn't use or wear, and chuck it. Packing light and efficiently will make your vacation much more enjoyable.
Remember to add your non-clothing items: non-liquid toiletries, spare shoes (if you must), and a security pouch or moneybelt. Liquid toiletries, prescription medicines, an extra pair of eye glasses, contact lenses or sunglasses, and your camera you'll carry with you on the plane in your carry-on bag, including a spare change of clothing in case your luggage makes it to a different destination than you do.
Your main suitcase may be of the size that can be carried on. Mine is and it's always been big enough--even for those 2 month trips to a remote archeological excavation.