I don't have any experience volunteering at conferences... but... I assume they mean 15 hours over the course of the conference (multiple days), not 15+ hours in a single day? Right?

The only advice I can think of... maybe plan out what you want to do. If you volunteer in the morning, figure out what afternoon sessions you want to do - and then make a conscious decision to get out "volunteer" mode and into "conference attendee" mode... basically have a plan going in, and know that you're going to stick to it... .

Good luck, and I hope you have fun and learn interesting things!