I hope everyone can read this and do likewise for graduation gifts. We heard it from someone, somewhere ten years ago and only do it now. (I just read over that and am convincing myself that I may be leading into some scheme; I'm not.)
1. Get them some blank, inexpensive card. Write some greeting in there about whatever, but with it include instructions to send you a letter from wherever there are in the next year (college, a job, new city, traveling the world during a gap year) with an address that you can write them back. (Feel free to tell them you'll send a care package if you'd like to divulge that information ahead of time.) We include a self-addressed, stamped envelope for them to make it even easier.
2. If they write you about life and thus give you an address to send a package, you go and get some practical food treats and whatever else, and send it to them. No, you don't give them cash (we sometimes include a gift card). No, you don't send them junk or some consumer sucka garbage. You send them whatever you want or think they'll like. Plus, you keep the connection you have to this student open.
3. If they don't ever write you, your sunk cost is maybe $1. If they lost the self-addressed envelope, they lost that opportunity and are one lesson closer to maybe being responsible.
Like I said, we've done this with about a 70% success rate. We hear back from them again and that it's one of their favorite graduation gifts because of the added personal touch and the delayed gratification. It may cost more to send the package than you'd originally intended to spend, but the overall cost evens out with the slackers who never write.