AN MU2 PLT FORGOT HIS ASSIGNED FREQ AND CONTACTED BFI TWR TO REESTABLISH COM IN S46 AIRSPACE.

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WAS BEING HANDLED BY SEATTLE APCH ON VECTORS FOR A VISUAL APCH TO RWY 31L AT BFI. ASSIGNED 5000 FT MSL; ON AN EASTERLY HEADING (L BASE FOR RWY 31L). GIVEN FREQ CHANGE TO ANOTHER APCH CTLR AND TOLD TO EXPECT LOWER ALT FROM THEM. WHILE TURNING THE RADIOS; INADVERTENTLY CHANGED FREQS TO BFI TWR AND GND CTL (TRYING TO PLAN AHEAD FOR THE FINAL) AND DID NOT REMEMBER THE APCH CTL FREQ. I REALIZED IMMEDIATELY I WAS OUT OF COM WITH APCH AND CALLED TWR FOR SOME HELP. I DID NOT CHANGE HEADING OR ALT UNTIL TWR CLRED ME FOR LNDG ON RWY 31L (I HAD RPTED RWY IN SIGHT). I WAS VERY CONCERNED BY THE FOUL UP ON MY PART; BUT CONCENTRATED ON THE FINAL APCH AND LNDG. WHILE TAXIING TO PARKING; WAS ASKED BY GND TO CALL SEATTLE APCH SUPVR AFTER SHUT DOWN. I COMPLIED. IN RETROSPECT; I REALIZED I WAS THINKING TOO FAR AHEAD AND SET MYSELF UP FOR THE MISTAKE BY NOT GOING STEP BY STEP. I BELIEVE THAT CONTACTING TWR WAS THE FASTEST WAY TO GET BACK IN THE SYS; AND I WAS VMC AT THE TIME; BUT IT WAS SCARY TO THINK WHAT WOULD HAVE BEEN THE CASE IN IMC. I WILL SLOW DOWN IN THE FUTURE.