One of the hazards of long-haul flights is that due to the extended periods of time spent in cramped conditions, deep vein thrombosis (DVT) can kick in (as mentioned by Rancid Pickle). Recent research has proved that this is far less likely to happen if you take an asprin before takeoff. This has the effect of thinning the blood and so reducing the chances of a blood clot forming (stroke victims take it for the same reason). This research has been taken so seriously that many airlines (including British Airways and Qantas) are soon to start handing out these pills prior to flights.
Regarding some of the other writeups in this node pertaining to alcohol consumption on planes. Personally I don't drink during flights as it gives me a huge headache and worsens my already screwed up sleep patterns, but if you decide to have a couple be very careful, as your alcohol tolerance plummets (due in part to the reduced oxygen levels in the cabin atmosphere), and being drunk on a aeroplane is illegal and you can easily get arrested... not a good way to start a holiday.
About half an hour after adding this writeup, I was reading New Scientist website, which had an article about DVT. It mentions the fact that some geneticists are suggesting that people be tested for a mutation in a gene called 'Factor V Leiden' which is related to blood clotting. The mutation occurs in about 1 in 20 people, but shows up in almost 50% of people hospitalised due to deep vein thrombosis caused by long haul flights. The article suggests that if people with this mutation could be picked out they could recieve extra water, and blood thinning drugs from the airlines.