Do I need an onward ticket?
Not only must you check your visa requirements, you must also make sure you have satisfied your onward ticket requirements as well.
What is an onward ticket?
An onward ticket is normalling proof of a pre-booked flight (or train) ticket out of the country you are about to enter.
Most countries insist that you hold a flight ticket out from their country which must be presented on arrival at immigration. If you do not have this you may be turned away with little choice but to get back on the next plane home.
If you think that this may be a problem, you must contact the relevant embassies to double check. There are however ways around this. If you do find yourself in an awkward situation where you wish to travel overland through countries which demand onward tickets you can do the following:
Travelling on the Trans-Siberian from Russia into Beijing and then wanting to travel overland down through China into Vietnam. If you can not pre-book the train down before you leave the UK you can buy a one-way ticket from China to another country and not use it. One of the cheapest on this route is from Kunming - Chiang Mai for about $100. You will then be able to claim much of your ticket back by canceling your flight when you are in the country and sending your ticket back to the UK for a refund. Obviously this can be a bit tricky but it is a possibility.
Other countries are more relaxed. If you do not have an onward ticket from their country they will often admit entrance if you hold an onward ticket from a neighbouring country. Travelling overland from Bangkok to Singapore is a classic example. Again this must be checked before you travel.