Money makes the world go around and without it you won't get far. You need to consider how much you are going to spend per day. How to carry it and what to do should it all be stolen or you run out.
The first decision is to decide how you are going to get around and what type of accommodation is available and what level of accommodation you will be using. The cost of getting around will vary considerably depending on what part of the world you are visiting. A good guide book will help give you some idea of the relative costs. Often buses are cheaper but a slower option than trains if you have limited time and are in a large country you may want to take an internal flight.
The cost of accommodation is very much a matter of choice however in some countries such as China the government may not let you stay in the cheaper accommodation so the cost may be out of your hands to some degree.
The other factor you need to look into is the cost of food. Eating cheaply is easy in most countries if you are going to buy your own food but if you intend to eat twice a day in restaurants then increase your budget. The cheapest way to eat in the developing world is usually from the street stalls that the locals eat from.
Once you've got a budget add a bit more on for emergencies or for that must have souvenir of your trip
When sorting out your money you have a choice of what currency you take. You may be able to get some currency for your destination before you go. This is a good idea but you'll often get a better deal when you arrive so change enough to get you to your first hotel, in general £50 ($75) will be more than ample. If the country you are heading to has high inflation then leave getting the your cash to the last minute. If you can't get currency before you leave then you will usually be able to get some cash at the airport on arrival.
The currency you carry might have a baring on the exchange rate and also the ease of exchanging your money. Safe currencies to carry are most Western currencies but in particular Dollars, English Pounds and French Francs are the best. The dollar is the best and taking a few dollars cash will get you out of trouble in most countries. The Pound and Franc are good since both England and France had large empires and these countries still have close ties with their former rulers.
Cash is the easiest way to carry money and will give you less hassle when you come to exchange it you'll also not get into having to pay multiple lots of commission. The down side is if it gets stolen you may will have difficulty getting it back. Don't carry any more cash than you are insured to carry.
Travellers cheques are the safest way to carry cash. The most important thing is to make sure that they are a well recognized brand American Express or Thomas Cook are the best. In some countries you may get a better exchange rate for travellers cheques. Travellers cheques are as good as cash and you'll find in many areas black marketeers give you a better rate for them especially if they have no signature at all on them. If you are travelling with a partner then you can get travellers cheques that can be signed by either of you although in some banks they may demand that both of you have to sign the cheque so it may be better to stay with separate cheques.
Always take a note of the number on the cheques you cash, make sure you have the emergency number and receipt on you separate from the cheques and leave a copy of the receipt and phone numbers at home with someone
Remember that the more remote the area you are in the longer it will take you to get a replacement cheques. So always make sure you have enough cash for a couple of days.
Increasingly you can use credit cards anywhere and they are always a good back up. If you use them keep the receipt for any transaction in case there are any queries when you get back home.
Usually the exchange rate offered is fairly good but check for extra charges before you leave home. If you use your card to withdraw cash there is usually a commission again check before you leave home and remember your pin number
If you are travelling for a period and have no one back home reliable enough to pay off your bill you can always send the credit card company money before you leave effectively over paying any balance. Then as long as you don't spend more than this amount you won't have to worry about interest payments.
Remember you card is very valuable check it's covered by insurance. Make sure you have the lost card number and give this number and your card number to a reliable person back home so they can help deal with the card company should a problem arise.
Whatever form you money is in you'll probably have to exchange it. In some countries rates and commissions are fixed while in others the rates will vary considerably. Check the rate and also if there is any additional commission or tax. In general you won't get a very good rate at airports and even in towns it is worth keeping an eye on rates as you travel around. Getting a good rate is a bit of a lottery the best rates are usually obtain on the black market if one exists.
If you run out of money it should be possible to get money wired out to you. If you are in the developed world then this should be fairly easy but in the developing world expect it to be a long tedious process. If you think you will need to get money wired out look into it before you leave.
In many countries you may have to fill out a currency form on your arrival declaring how much money you are bringing into a country. It is essential that you keep all exchange receipts as they may be required when you leave the country they may or may not ask you to count out the money you are declaring and when they start asking for a 'present' pretend not to understand. The customs officer can then check the receipts against the cash you have left and if there is a discrepancy might 'fine' you. In some countries you will need receipts to change back cash before you leave
If you don't declare all your cash then you can exchange what you haven't declared on the black market. However remember you and your gear may get searched and you'll lose any unaccounted for money. It is also necessary to change enough money officially so that it shows you have lived off it. No customs official will believe you lived of £10 for two weeks. In some countries it used to be necessary to change a certain amount of cash for every day you stay there. Check this when you enter and it might be worth getting something official in writing especially if you are planning to leave by a small border town where the customs may try all sorts of tricks to get a few dollars out of you.
In some countries it is illegal to export their currency and even if you do you'll find it difficult or impossible to exchange it back home so change it back before you leave it's better to have a poor rate than no money. Check you haven't got to pay any airport tax first though.
Large western hotels in the developing world may only except a strong currency such as the dollar.
The best thing is to split it up into lots of separate places, money belt, hidden pockets, rucksack, daysack and in your hotel room. If you are travelling with someone you can trust then get them to look after some of your money (stick to travellers cheques). They will probably be only to happy to let you look after some of theirs.