I used to be in the camp of attributing hangovers to simple dehydration. Apparently the data doesn't really support that. Interesting reads on the subject always come up on 1/1 of any year!
Originally Posted by linked article
The most compelling theory, at the moment, is that hangovers result from a buildup of acetaldehyde, a toxic compound, in the body. As the body processes alcohol, acetaldehyde is the very first byproduct, and it’s estimated to be between 10 and 30 times as toxic as alcohol itself. In controlled studies, it’s been found to cause symptoms such as sweating, skin flushing, nausea and vomiting.
Hangovers could also be driven by the way alcohol messes with your immune system. Studies have found strong correlations between high levels of cytokines—molecules that the immune system uses for signaling—and hangover symptoms. Normally, the body might use cytokines to trigger a fever of inflammatory response to battle an infection, but it seems that excessive alcohol consumption can also provoke cytokine release, leading to symptoms like muscle aches, fatigue, headache or nausea, as well as cognitive effects like memory loss or irritation.
Even so, my methods probably won't change. Mild OTC painkillers, lots of water, and time.