Round the World Flights
If your route takes in two or more continents, consider a round the world ticket as these can be be quite economical compared to individual flights. Travelbag is a good site for round the world tickets and it also has a route planner. Many round the world tickets such as those offered by Star Alliance, One World and other schemes offer the flexibility to change dates at no cost or minimal cost. Changing the route will incur high additional costs.
If your route covers one continent, a combination of flights & travelling overland by train & bus from country to country may work out better. Other flight companies to consider are Opodo and Ebookers. For students, STA Travel may offer budget or low cost flights.
Long haul flights to the other side of the world can come as a shock to many people with many travellers suffering from cramped seating with very little leg room and poor quality food complimented by even poorer quality toilets. Although it can be tempting to overdose on alcohol to numb the pain, this only makes things worse. The general advice here is to drink lots of water and take occassional walks (preferably within the airplane) to help avoid deep vein thrombosis (DVT). Exercising the lower legs discreetly while seated and wearing graduated compression stockings (which are all the rage across catwalks this summer) can also help.
Invariabley, crossing various time zones in a long haul flight may also lead to jet lag and a general feeling of tiredness when you arrive at your destination. It's best to take things easy for the first few days as your body clock adjusts to the new time zone.