West do the Washington side, ferry over to Oregon, backroads to Clatskanie then backroads to Vernonia. From there, take bike paths and back roads to Portland. Perfect route comes and goes.
What John said. This route is what the new Cycling The Pacific Coast book by Bill Thorness recommends. It's a lot better than 26 or 30, the two ways the old Kirkendall/Spring version used. There will be three climbs: 47 out of Clatskanie, the summit on Banks-Vernonia Trail (not so bad since it's a rail-trail) and going over the West Hills into Portland. Most Portland people will skip riding through the Tualatin Valley and catch the MAX light rail from Hillsboro into Portland.
An alternate is to catch the Crown-Zellerbach Trail from the "town" of Pittsburg, a few miles north of Vernonia on 47. It's another rail trail, but is unpaved and still a bit rough in spots. It'll drop you off in Scappoose. Then you can take 30 the rest of the way into town. The trail is nice, riding on 30 from Scappoose to Portland not so.
Below is a pic of the route map the Thorness book uses.