Think about what time of year you're traveling. Winter is cold and wet, summer is fairly mild, though it can get quite warm.
Remember that the "layered" look is always in and is quite a practical option. A sweater over a shirt with a jacket will allow you to take layers on and off as needed to adjust to changes in the weather.
Keep in mind, too, that Portland is the home of Nike and Columbia Sportswear headquarters. Tennis shoes and sports gear are regular attire in this corner of the Northwest. Flannel shirts are welcome here, too.
Be prepared for rain. If you have a waterproof jacket shell that you can throw in your bag, you'll appreciate it if the sky opens up.
Be aware that Portland is pretty casual and even some of the nicer restaurants are full of folks in khakis. Business casual has become a way of life, so don't hesitate to bring a suitcase full of clothes you find yourself most comfortable in.
