"Passport, digital camera, and wallet. Everything else can be replaced."
But here's some more realistic tips on packing your bag. Firstly you are a budget traveler, your "bag" is probably best going to be replaced with a backpack, much more versatile, much easier to get around and switch places on the go, we are budget travelers, going to different countries on trains, buses, and dhows! We don't have time to pack luxurious bags and load them into private cars for hire. I use this one, it's gotten me through the 6 continents, I crossed Africa with it, my friend also shares it, it's been to Israel, London, Colombia, numerous places in Asia, etc and recently it participated in the running of the bulls in Spain, unknown whether or not the bulls ran it over, but it came back in mint condition. It's got multiple pockets and can be used as a small or large bag by adjusting some straps:
So you have your back pack now and have replaced the bag.
-Consider the trip you are going on, if you are going on a camping trip in Africa there will be no need to pack shoes or dress shirts, but you may need things like biodegradable laundry, or some camping type soaps to wash yourself.
-Minimize everything! When I go on a trip I am very likely to buy clothes where I am going thus that is one less shirt or one less pair of whatever that needs to be packed, you want your bag to always be light and manageable. Bringing 20 outfits for a 7 day trip is not going to be a good idea. Always remember you are likely to come back with more than what you left with, so leave room in the bag for souvenirs etc.
-What cannot be replaced? Besides the golden rule, passport, wallet, and digital camera, what can you not buy where you are going that you will need? Medication? Contraceptives? Your cell phone? A lap top? Consider where you are going to?
-Leave some of it there if you can. I am going to New Zealand soon and will be hiking the Waitakere Ranges in rain and mud, that old pair of hiking sneakers I am bringing to Auckland, will be staying there when it is all done. As will some of my clothes if I need room to bring back some wine.
-Don't forget if you are going international, you need entertainment! International flights are a killer, bring your books, mp3 player, or whatever, I always bring paperbacks because I read fast and will usually trade off my books or give them to a fellow traveler. Always have a pair of earphones, it costs you 99 cents outside the airport and in the airport those war criminals will charge you up to $20!!!
-Your carry on bag, should fit in your main bag. I always bring a small back pack for day or over night trips if the large backpack is gonna stay at the hotel, but it fits in my large backpack. If this doesn't make sense now, it will when you are traveling. Trust me.
Here's a rough list of things you may need to pack, it's a unisex list, so if you are a male you will not need to bring tampons... some of you anyway...
1. ID’s, passports, travel documents, maps
2. MP3 player
3. Mini Speaker System (if you plan on having parties in your room, this comes in handy!)
4. Alcohol - where are you going? Will they have booze available
5. Razor and cream
6. Cologne (mini bottles)
7. Hair styling products/combs/etc
8. Make up (it's ok guys, we know some of you wear make up too)
10. Tooth brush/paste
11. Camera, digital camera, underwater camera
12. Camera batteries/charger
14. Pens, pencils, paper
15. Book for flight/down time
16. Sun tan lotion
17. Sun tan oils (some of you tan hard)
18. Aloe Vera (some of you burn easily)
20. Caffeine drink
21. Snacks/food (if applicable)
22. Swim suit
23. Foot spray (you know if you're the type who needs this one!)
25. Cell phone and charger
26. Car charger for cell (if applicable)
27. Glasses or contacts
28. Eye Drops
30. Beach Towel
31. Coupons (if applicable)
34. Reciepts/flight info/TICKETS
35. Ear phones/head phones
38. International charger
41. Sleeping bag/tent/ camping gear if needed
42. International Driver's License if needed
43. Lock for Hostel Locker
44. Business cards/contact cards, great for meeting other travelers
45. Immodium/Anti diarrhea medication depending on the destination.
46. An extra roll of toilet paper if it's a place where you think you may need it
47. Clothes... duh....