I am still working on this myself and by no means am an expert. Here are some ideas that worked for me:
1. Try to make flexible "base" recipes such as a curry or stirfry with whatever vegetables you have or "wilted greens" - I use the same recipe for all of them (sochan, collards, kale,
wood nettles, radish tops) - sauté in olive oil with a little garlic, don't overcook, add vinegar and salt towards the end.
2. Gorge on a particular food while in season and try to avoid it out of season. You get tired of the food and don't really crave it until it's in season again. I don't really eat out of season tomatoes anymore (aside from occasional canned ones). You can alter the recipes to do without.
3. Plan for the hungry gap. This period is tough. This is when I usually cut myself some slack and buy more things that in season in warmer places (i.e. citrus or avocados). I am now planting MORE things that I can harvest in early spring vs. planting a whole lot of stuff that ripens in August.
4. Things that grow together often go together when cooking (ramps and
morels, for instance).
5. Don't forget to forage. This is particularly useful in the hungry gap of spring.
6. Preserve your harvest. Fermented food is always "in season"! (
Wild Fermentation by Sandor Ellix Katz is a good book). I also recommend the Everlasting Meal Cookbook by Tamar Adler. This book is more about leftovers, but it gave me other ideas on how to cook "without recipes". Foods that store well at room temperature (drying beans, certain types of squash, garlic, onions) are also in season pretty much the whole year because of how long they last. We also dry a lot of things. I hate canning so I don't do it, but again it's a great way to preserve food.
7. Make yourself a challenge to eat something from your garden every day (if you don't have a garden, it could be "eat something local"). It could be something as simple as some parsley.
8. Big part of eating seasonally (and also locally) is you get to look forward to things. Essentially, by "depriving" yourself of foods shipped from far away, you will start to look forward to things that are in season. For instance, I no longer eat mango, but I really look forward to when raspberries ripen. I would totally eat mango, of
course, if I could grow it. The point is when we become accustomed to extreme variety at the grocery store, we may not value seasonality. On the other hand, once you start eating things in season, you start to realize many things at the grocery store don't taste good (those awful tomatoes...).