Steps

1. Get a small bag. The bag should be lightweight and easy to carry. It is helpful to have a bag that can be carried as a backpack, or as a shoulder bag depending on the circumstances. It is also handy if it has wheels. It is quite common to have to walk long distances, climb stairs and have to do many things at once, all while you have your bag with you.
2. Determine the climate conditions of your destination. Check the weather report, so that you know what type of clothing to bring with you. For example, if the weather is going to be warm, then there is no need to bring a heavy jacket. If it is going to be sunny, then bring a hat, etc.
3. Plan your wardrobe so that everything matches with everything else. This does not have to be monochromatic or boring. Rather, it suggests a minimalist approach to fashion. One skirt and a single pair of trousers can be matched with a t-shirt, a fancy blouse or a tank top to make six different outfits--plenty of clothes for a week journey. You can get fancy with a scarf, bandanna, hat or other uniquely "you" wardrobe piece. Also, be sure you have appropriate clothing for area you are visiting, and for holy sites if necessary. If done right, you might be able to get away with one load of laundry rather than 'darks' and 'lights'.
4. Bring enough clean underwear so that you can get through at least half of your journey's time without doing laundry, up to 10 days time. After that, you will have to do laundry anyway. This also goes for socks, if you will be wearing those daily. And, bring laundry supplies, if you are planning on doing laundry. (Note: just put enough dry detergent in a ziplock bag for as many loads as you plan to do--don't bring the whole box.). Alternatively, if you are not moving much you can pack 3 days worth and regularly sink-wash them, allowing about a day for them to dry.
5. Bring at least two pairs of shoes that you can walk in comfortably. In the event that one pair gets wet, or your feet hurt unexpectedly, you will be incredibly grateful that you have that extra pair of shoes. You can wear one and carry the other. I also suggest bringing a fun or fancy pair of shoes, in case you want to go someplace like a club, a fancy dinner, or the opera. You never know who you may meet!
6. Consider your packing space carefully. Put items inside of other items, like those Russian nesting dolls. Roll clothes into small rolls to reduce wrinkling.
7. Pack your dry toiletries in the same bag, and your liquid and gel toiletries in another same bag. If you are going to be flying, then your liquid and gel toiletries must fit into a 1 quart/liter ziplog bag, and no one of the bottles can be more than 3 ozs.
8. If you have special supplies or equipment that you need to have with you, invest in high quality travel versions that fit into a smaller bag. Another alternative is to have these items sent ahead of you to your hotel or home stay where you can meet them later. This sometimes takes a bit of planning, but it is much easier than carting items around with you.
9. Keep your hand lotion, water, sunglasses, ear plugs, snacks, lipstick (or chapstick), toothbrush, floss, hair brush/comb, a change of underwear, pen, stickee notes, and any special medicines and necessary items you need in the top of your bag or in a separate hand bag, because you never know what may happen when you travel. As well, carry any valuables with you in this bag (e.g., camera, computer, jewelry, etc.)
10. Keep your ID, money, credit cards, in a security pouch. Keep some small currency in your pocket for little purchases (rather than pulling out everything each time). Carry photocopies of your ID, visas, and credit cards with you but in a separate compartment from the real ones (the idea being that if you lose one, you still have the other). Copies can also be scanned and kept on a USB thumb drive (possibly encrypted). Consider 'buddy packing' where your travel partner keeps your important photocopies and you keep theirs.