I agree with all of the above posters. Most importantly, you should be professional. Dress like a professional and be very courteous. Let the OD do their job when with patients and save your questions for when they are not with a patient. Definitely thank patients that allow you to observe their exam. Take time to shadow everyone at the office, not just the doctor. Spend time with the technicians and opticians and front desk staff.
Send a thank you card to the doctor and staff after your observation time is over... you never know who may be hiring 4 years down the road (especially in a saturated city like Portland), it's never too early to leave a great impression.
Do leave the doctor a copy of your CV/resume and a cover letter explaining why you want to pursue optometry and any other notes about yourself so the OD has some information about you he/she can pull from when you phone to ask for a letter of recommendation. We can't remember everything about everyone, and a reminder is appreciated.
I can't stress enough the importance of shadowing OD's in multiple different settings. Make sure you shadow OD's in private practice and commercial practice. Being that you're in Portland, shadow an OD at the VA hospital, and shadow an OD at the Casey Eye Institute. You may even want to shadow an attending doctor at Pacific.
Remember you are there to learn and decide if optometry is for you. Be a sponge.