A few things that may help:
Plenty of rest the night before (and no alcohol)
Avoid long stressfull journeys to the airfield if possible - you'll be knackered before you start!
Drink plenty of water (dehydration's quite a major contributor to fatigue and headaches. After a hard sortie it's quite likely that your back will feel drenched due to sweating)
Eat appropriately - I'm not a dietician but I know that some foods will be better than others (e.g. foods that are slow release, such as bananas, rather than quick release energy bursts such as candy bars)
Remember to switch into flying mode and don't allow non-flying thoughts (e.g. work, family, etc) to distract you. Switch the mobile off when you arrive at the airfield! This helps to reduce your stress
If you wear glasses, keep them clean (and have your eyes retested to make sure that your not straining them)
Finally, remember that you're under quite a lot of mental pressure, with a high level of continous concentration, and your body will be experiencing some unusual physical sensations (e.g. G-load when turning). It takes time to adapt and for your body to acclimatise to this new regime.....but it's well worth it!