I have made a little primitive map which shows where the best places to find Razor Clams are. If you want to go to Westhaven, I recommend following the map to Westport, then turning toward Westhaven State Park on Jetty Access Road. Park at the state park and walk down to the beach.
To try Grayland, you can access the beach either on Bonge Road or Grayland Beach Road. The area in between these two roads are prime razor clam habitat.
Good luck & happy Clamming!