ODFW has a lot of good info- http://www.dfw.state.or.us/MRP/shellfish/razorclams/index.asp
Check out this video: http://www.youtube.com/watch?v=rA4D2ERtTdo
Sometimes you will see those “shows” on their own, especially right at low tide. I like to stomp my feet as I walk in about a ten foot circle and then walk back over my tracks and look for shows. As the stomping scares the clam it will retract and cause a show.
Here’s one of my blogs- http://www.ifish.net/board/blog.php?b=23&a=3956
Get to the beach one to two hours before low tide. Minus tides are what you will want.
http://www.saltwatertides.com/dynamic.dir/oregonsites.html The Columbia R entrance or Seaside are the ones to look up for the Clatsop beaches.