Travelling by yourself is probably more exciting than going with friends, but make sure you keep people abreast of where you will be. This isn't always possible down to exact location, but the general area or region means that if you are uncontactable for a few days, rescue services at least have a starting point.
Make your travelling companions, tour guide etc aware of any allergies you have and if you start to feel ill give them a heads up. It may just seem like a cold, but malaria and other tropical diseases also start in the same way.
Don't flash your camera, phone, MP3 player etc around. To really feel like a backpacker, you will need to fit in with the locals, so don't draw unnecessary attention to yourself. Also, don't walk about reading a guide book and try to learn at least a little bit of the local language as this will score you valuable points with the locals and it will also make you look like you have been there longer than 24 hours.
Practical information:
Apart from carrying local currency, it is a good idea to have some US Dollars with you. If you ever get into difficulty, particularly in countries in Africa, Asia and South America, US currency will get you a lot further than anything else.
Photocopy your passport and keep it separate from your actual passport. If you do lose your passport or it gets stolen it's much easier and quicker to get a replacement if you have a photocopy with you.
If you are going to be in any one country for a long time, then buy a local pay as you go sim card. This means that mum and dad can call you and you won't have to foot the bill plus phone cards can also be used from these phones in many countries too. However, be sure to check that your phone is 'unlocked' before you leave the UK.
Planning:
The best and cheapest way to see as much of the world as possible is to buy a round the world flight ticket. These are also some of the most flexible tickets you can buy where changing dates and flight routes will only set you back a minimal admin fee of around ?25. If you were to buy normal return, or one-way tickets it will set you back a lot more of your hard earned cash.
Some things are best done locally. This not only helps support local communities in the countries that most need it, but you can save your pennies and retain much more flexibility. This is applicable to tours and accommodation, but also to things like toiletries. Shampoo, deodorant, sun cream etc are heavy, and rather costly in the UK. So save luggage space as well as cash by buying them when you arrive at your destination. Speaking of luggage, I'm sure you have heard stories about luggage being lost for days, so just in case this does happen to you, keep a spare change of clothes and any medical supplies in your hand luggage.