You know how hair stylists sometimes ask if you'd like to book your next appointment right after getting a haircut? If you know you're going to be busy, this is something that could possibly work between friends. If you're already out with them, talk about the next time you'll see each other. You don't have to be aggressive about it, just try not to be so vague. Another thing that helps is Google calendar (don't forget to set reminders if you're a busy bee).