I've never had to address this on sea-level hikes, or in cooler climes, but was honestly amazed that my usual insatiable appetite was gone for the first few days in the Sierras. Not only does not eating affect your energy levels, ability to stay warm, and sleeping patterns, but you have to carry that extra food for the rest of your hike!
Some of my own tips, and some suggestions from other backpacker.net forum posters:
- Pack accordingly: lighter dinners for the first few nights, progressing to more satisfying meals once you aclimatize.
- Meals that work as leftovers: I usually make burritos my first night, then if I don't eat them, they make a great cold breakfast snack.
- Pack tasty food: If you don't truely crave the meal at home while packing it, you probably won't crave it on the trail: only pack your favorite meals, especially for the first few nights.
- Stay hydrated: bring juice crystals, koolaid, iced tea, whatever will encourage you to drink more water.
- Get to camp early: Spend some time relaxing in the shade, swimming, slowly drinking, and your appetite may return. Nibble on some trail mix or other easy to eat snacks.