I have a grate pet hate of people taking large wheelie suitcases as carry on only for them to arrive and pick up a huge case in checked luggage, unless you are only taking carry on, a suitcase is way too much. The bag you take on needs to be small enough to fit neatly into a small space but big enough to hold everything. I have an obsession with 60's/70's weekend bags with them being the perfect size for traveling but can be used for a short long weekend trip. A shoulder strap can keep your hands free or if it only has handles make shore that they can be comfortably hooked over your shoulder. My bag is a rip curl overnight bag that is large enough to hold at least 3 days of clothing and is waterproof outer so you can wipe it clean.
A good idea is to take one change of clothes with you on the plane in case your luggage gets lost in transit and it takes a few days to get to you (this happens quite a lot) you can at least mix and match what is in your carry on with what you wore on the plane. A small makeup bag is also a good thing to have with you that contains some moisturizer and hand cream in case your skin gets dry, a lip balm, concealer, toothbrush and tooth paste so that you can be a little refreshed when you get to your destination. If you are on a train or bus some sunscreen is also a good idea.
Entertainment is a must for going anywhere long distance. iPod, laptop, camera, and books are handy. If you are crafty like me and need to keep your hands busy some sewing is quite handy or a plain exercise book to draw in. A healthy snack is a good thing to pack if you get peckish but if you bring fruit and you are traveling between states in Australia or from overseas into Australia remember to put it into the quarantine bins when you get to your destination to avoid paying a large fine.