Prepare an itinerary, but do not lock yourself into a rigid itinerary too soon. Give yourself the flexibility to make changes as you desire.
Determine the total amount that you want to spend. Then proceed to budget this amount for the various expenditures: transportation, lodging, meals, entertainment, gifts, books, side trips, etc.
Get your passport. Make photocopies of the identity page of your passport and air ticket for separate safekeeping. This will facilitate easier renewal should your passport be lost or stolen. A passport is your best form of identification and comes in handy for cashing traveler's checks.
Check on whether or not you need a visa for the country(ies) you'll be visiting.
Study the cultures of the places you will be visiting including the language, arts, geography, history, customs, etc.
Check your calendar for obligations you might have during the time you'll be away. Take appropriate action.
Determine clothing and items you may need for your trip. You may want to take less clothing with you and buy some items locally, at your destination. You can see what the locals are wearing and blend in with them, instead of looking like a tourist.
Draw on your past experiences (and others) to make up master lists of wardrobes, toiletries, health needs, miscellaneous.
If you plan on buying items abroad, pack your clothes into a medium-sized suitcase and pack that suitcase into a larger one. The larger suitcase can hold all your purchases on the trip home.
Get your financial means together. Check expiration dates on credt cards, get your traveler's checks, etc. You may want to prepare a foreign currency "packet" which contains just enough foreign money to take care of your immediate expenditures upon arrival for taxis, buses, tips, etc.
Leave copies of your itinerary with family members and friends. Don't promise anyone you'll call them as soon as you arrive. Remember, telephone service, and especially long distance telephone calls, may be too difficult for you to arrange when you first arrive. If you're planning on doing even a modest amount of long distance calling, get an AT&T International Calling Card.
As you purchase items abroad, record their cost. This will save you time in filling out your home country customs declarations.
If taking expensive electronics or cameras, register them with customs at your home airport before leaving the country(so you don't have to pay duty on them when bringing them back home!)
Store your valuables defensively (in interior pockets or money belt). Don't use back pockets or purses which can be snatched by joggers or bikers. Necklaces, earrings, and watches are also favorites of snatchers. If you really want to play safe, leave your jewelry at home and only wear a cheap, inexpensive watch.