Here are some things that I do:
Choose clinicians carefully. I’ve found one that I really like, and I’ll be sticking with that person when he’s back in the area. That said, I’ve done two clinics in four years (spectated others).
Go schooling and showing at venues that are as near by as possible.
Take lessons, but not necessarily weekly. I’m taking one or less a month and really practicing in between. I’ve found that I value the lessons that much more and practice that much harder. If I do get stuck wrt to something, coach and I email (only when really needed.) That said, I have in the past been in “full training” with five lessons a week for months on end, so I do have that to draw from. It depends on where you are at with your riding and your horse.
Work in trade for lessons or riding time.
Go to shows to see where your skills are at: it’s mostly because I’m really short on the money, but I only went to a few shows, and based on results there I know exactly what I need to work on, which I’ve been doing for the past three show-free months. I expect a big improvement next time I get out.
And of course the obvious (but pretty darned hard to do, dang it): buy everything used. The only item I’ve bought brand new in recent memory: 1 pair of breeches, 1 helmet, gloves (but those were a gift). I borrow x-c vest, I borrow show jacket, since I’m not using them every weekend anyway. Just dry cleaning cost for jacket.
Volunteer. Aside from gas money, it’s free, and educational.
Read. It’s free and educational.
Learn to be good at dressage so that you can work your horse in a way that keeps him sound with little to no “maintenance”: injections, Adaquan, chiro. Not always possible but the right kind of riding and fitness work will at the very least help you to avoid vet bills.
Practice good stable management and veterinary preventative medicine to keep horses sound, healthy, happy and most importantly rideable.
FWIW I lease, I don’t own, no truck or trailer, and I’m over forty. I’m hoping to get out more next year, already scheming to make it happen. I’m not the typical eventer but I am managing to be a part of the sport and I do ride 5 or 6 days a week.
In a certain way, I save money by seeing eventing as a way to develop my horsemanship rather than a sport that makes showing all the time imperative. If I had more money would I have gone out more? Probably, but I’m not sure that it would have improved my riding. So if improving the riding is the goal there are certainly ways to do that as an eventer.