A BE99 SINGLE PLT; FLYING CARGO; MADE AN ALT OVERSHOOT WITH NCT TRACON.

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FINAL LEG (MOD-OAK) ON 135 CARGO FLT WITH A FAMILIAR RTE. RADAR VECTORS TO JOIN AIRWAY; CLB AND MAINTAIN 6000 FT. AFTER DEPARTING MOD; HANDED OFF TO NORCAL APCH CTLR AND TOLD TO CLB AND MAINTAIN 5000 FT. I READ BACK 5000 FT AND SET MY ALT REMINDER BUG FROM 6000 FT TO 5000 FT (JUST INDICATES VALUE; NO WARNINGS). WHILE CLBING THROUGH ABOUT 3500-4000 FT; I BEGAN CONCERNING MYSELF WITH THE INEVITABLE APCH INTO OAK. I KNEW THE WX WAS IFR AND I BEGAN PREPARATION TASKS FOR THE APCH (PICKED UP ATIS AND LEAFING THROUGH THE CHARTS IN MY BINDER). I PASSED THROUGH 5000 FT AND CONTINUED TO 6000 FT; THE ORIGINAL ALT. UPON REACHING 6000 FT; THE CTLR GAVE ME A VECTOR FOR TFC AND REMINDED ME THAT I'D BEEN ASSIGNED 5000 FT. I FELT SICK TO MY STOMACH AS I REALIZED MY ERROR. I TURNED TO THE NEW HDG AND TOLD THE CTLR I WAS DSNDING TO 5000 FT IMMEDIATELY. HE AGREED THAT WOULD BE DESIRABLE. THE ALT BUG INDICATED 5000 FT AND I REMEMBERED READING BACK 5000 FT. ABOUT 5 MINS LATER; THE CONFLICTING TFC PASSED AT MY 3 O'CLOCK POS; MAYBE 10 MI AWAY. TFC WAS NO FACTOR; BUT THAT WAS DUE TO THE CTLR'S VIGILANCE AND PRE-EMPTIVE ACTION. THE CTLR ASKED ME IF I'D READ BACK 5000 FT TO HIM. I TOLD HIM; YES I HAD; THE MISTAKE WAS TOTALLY MY FAULT; AND I APOLOGIZED FOR CREATING THE SIT. THE CTLR SAID NOTHING MORE ABOUT IT; HANDED ME OFF TO THE NEXT CTLR; AND THE REST OF THE FLT PASSED WITHOUT INCIDENT. THE LESSONS I'M TAKING AWAY FROM THIS INCIDENT ARE NUMEROUS. YOU MUST CONSTANTLY GUARD AGAINST COMPLACENCY. I WAS COMPLACENT ABOUT MY REGULAR CLRNC; NOT CHALLENGING MY PRESUMPTIONS; AND TAKING THE PHASE OF FLT FOR GRANTED. I GOT AHEAD OF MYSELF AND BECAME PREOCCUPIED WITH MY APCH AND DIDN'T PAY ATTN TO WHERE I WAS AND WHAT I WAS DOING AT THE MOMENT. I NEED TO CHANGE SOME PROCS OR AT LEAST INCORPORATE THINGS MORE EFFECTIVELY. THE ALT BUG WILL BECOME AN ACTIVE PART OF MY SCAN AND NOT JUST USED PASSIVELY FOR REF. I WILL CHALLENGE WHAT I'M DOING MORE AND REMIND MYSELF '1000 FT TO GO.' MOST IMPORTANTLY I NEED TO PUT FIRST THINGS FIRST; NOT GET TOO FAR AHEAD OF MYSELF; AND FOCUS ON THE TASK AT HAND. I NEED TO ASK QUESTIONS AND GUARD AGAINST COMPLACENCY.