Eat a proper meal before you start drinking. During the sesh, mix in glasses of water between drinks. Try and have something to snack on during the sesh or at the very least, at the end of the evening.
Before bed, mix Grandpa and a sachet of electrolyte powder in to a glass of water. Have another glass of water. This can help to further reduce dehydration and beat the headache before it arrives. Have something like Bioplus or any other vitamin/energy powder or effervescent you can mix in to a glass of water for when you wake up. Fruits to eat, bananas for the potassium, magnesium and natural sugars are a good option. Hard boiled sweets to suck on to help replenish sugar levels are handy as well. Have another glass of water.
Basically, do whatever you can to reduce the effects of dehydration while helping your body replenish what it loses to alcohol.