If you're not coming from wealth:
Apply for free/cheap festivals. The more accessible ones include Texas Music Festival, Round Top, NOI, Colorado College Music Festival, Sarasota Music Festival, Festival Napa Valley, NSO SMI (if you're young enough).
You can always apply for Aspen and just not go if they don't offer you enough money. You can even tell them outright that you're not willing to pay that much and try to negotiate a better offer—especially if you're also accepted to one of those other festivals.
If you don't get into any of those, spend your summer getting lessons with an excellent teacher who you have not regularly studied with before. Ideally a principal player, just because principals hire subs, so this is a good way to start cultivating the roots of a relationship that could get you gigs later. If you're back in your hometown staying with your parents, playing for the local principal player is a great idea because you can (hopefully) stay with your parents for free whenever your hometown orchestra calls. As a freelancer, it's sometimes hard to give up work to take a trip to visit family, so getting paid to visit family is a great arrangement to cultivate. (Maybe also get a part time summer job, but it's hard to have the energy to practice if you're working too much. Something like 30+ hrs/week is probably too much.)
Once you've gotten into one of the festivals above, start also applying to a couple of "reach" festivals each year: Music Academy, Tanglewood, Verbier, PMF, NRO... Getting accepted to any of these before you finish school is a major sign that you're on the right track.
As you transition from student into professional life, keep in mind that sticking around town during the summer is a great way to start getting your name out there in a freelance scene. Lots of highly-compensated orchestras have summer seasons where many of the regular players take time off, and even many of the other local freelancers also go off to festivals or on vacation, so the summer season is a great time to get your foot in the door (LA, Cleveland, etc.). Even if you're not getting called by the big orchestra in town, random gigs will still come your way where you'll meet other people who can get you more gigs later.