CAPT NOTICES FUEL IMBAL; BEGINS TRANSFER. DISTR AND FORGETS; TRANSFER CONTINUES TO CAUSE OPPOSITE IMBAL.

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FLT MCI PHX B737-300. SUBJECT: UNINTENTIONAL TRANSFER OF FUEL BEYOND THE AMOUNT NEEDED TO BAL WING TANKS. WHILE AT FL310 ON THE ABOVE MENTIONED FLT. JUST PRIOR TO TOP OF DSCNT; THE CAPT BEGAN TO TRANSFER FUEL TO BAL OUT THE WING TANKS. I WAS UNAWARE AT THE TIME THAT HE HAD STARTED TO BAL THE FUEL; AS NO VERBAL STATEMENT WAS MADE TO ME AS I WAS ON THE RADIO TALKING TO ZAB. THE CAPT WAS THEN DISTRACTED BY A FLT ATTENDANT COMING INTO THE COCKPIT TO ASK A QUESTION. WE WERE FLYING INTO A BRIGHT AFTERNOON SUN; AND HAD USED THE CHKLIST AS A SUN SHADE INSTEAD OF PUTTING IT BTWN THE TWO THROTTLE LEVERS AS A REMINDER THAT FUEL XFEED WAS TAKING PLACE. WE WERE ARRIVING INTO PHX DURING THE AFTERNOON 'BANK' OF FLTS AND OUR WORKLOAD BEGAN TO GREATLY INCREASE. I GOT BUSY TALKING TO COMPANY ON THE #2 RADIO AND THE CAPT BECAME DISTRACTED FROM THE FUEL IMBAL; AS HE WAS BOTH FLYING THE ACFT AND HANDLING THE RADIO WORK WITH ATC. ATC STARTED GIVING NUMEROUS CLRNCS TO USE; IE; SPD AND ALT RESTRS AND SOME HDG CHANGES. WE LANDED UNEVENTFULLY AND UPON CALLING OUR 'ON AND IN' TIMES TO COMPANY AT THE GATE; THE CREW NOTICED THAT THE FUEL IMBAL HAD EXCEEDED THE AFM LIMIT OF 1000 LBS. WE HAD BEEN FLYING NIGHT TRIPS THE FIRST 2 DAYS OF OUR PATTERN; AND NOW WE WERE BACK ON A DAY TRIP. I FEEL FATIGUE CONTRIBUTED HEAVILY TO THIS FUEL IMBAL OCCURRENCE.