Your pre-departure costs will include flights, visas, immunisations, a medical kit, travel insurance and equipment. Estimating what you'll need while you're away depends on many factors. Will you be working? Will you be travelling on your own or with company to share the costs? Are you intending to flit around or hunker down for a while? One rule of thumb is that every day on the road can cost twice as much as a day in one place.
Parts of the world where your bucks can be stretched include southeast Asia, the Indian subcontinent, China, Mexico and Latin America. Parts where they can't include Australia, New Zealand, the Pacific, Europe, Russia, Japan, North America and much of the Caribbean. Africa can be cheap but some countries, such as those using the West African CFA currency (eg Senegal and Mali), are surprisingly costly. Write down where you're going and how long you're going to spend there and use Lonely Planet's Destinations section to work out your average daily spend. Don't forget to budget for emergencies and the occasional splurge - you don't want to spend your whole time wearing sackcloth and eating lentils.
There are three main student or under-26 cards on the market: the International Student Identity Card (ISIC), the International Youth Travel Card (IYTC) and the Euro<26 youth card.
These days there are good deals to be had on international flights regardless of your age; however, these cards can often secure you better ones. All the cards will also get you discounts at museums, restaurants and hostels. A YHA Membership card entitles you to stay at any of their 4500 youth hostels around the world.
The best approach is to take a good mixture of credit and debit cards, travellers cheques (if your destination is short on ATMs) and old-fashioned bank notes.
Do remember to keep your exchange receipts in case they're needed as proof of legal exchanges either within a country or when leaving. Also, if you're travelling with a mate or a group of friends, divide your money up: if one of you is robbed, the other can step in with the cash.
If you plan on using your credit card overseas don't forget to inform your bank first. Many banks will suspend transactions when they see the card being used internationally.