B757 FLT CREW HAS A TRACK HDG DEV DURING CIVET ARR TO LAX.

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WE WERE CLRED TO DSND ON THE CIVET ARR PLANNING RWY 24R. WE HAD ALREADY BEEN GIVEN ONE SPD RESTR TO 250 KTS. WE WERE PROCEEDING DIRECT TO CIVET WITHIN ABOUT 10 NM WHEN WE WERE SWITCHED TO RWY 25R. WE HAD ANTICIPATED THIS AND HAD BRIEFED IT AND HAD LOADED IT INTO RTE 2. THE FO WAS FLYING; SO I ACTIVATED THE CHANGE AND VERIFIED WE WERE DIRECT CIVET FOR RWY 25R. WE WERE ALSO TOLD TO LOOK FOR TFC; A B737 APCHING FROM THE S PASSING AHEAD FOR THE N COMPLEX. WE WERE RESTR TO 15000 FT BECAUSE OF THE TFC PUTTING US BEHIND IN THE DSCNT PROFILE. AT THIS TIME ATC GAVE US A FURTHER SPD REDUCTION OF 210 KTS. SINCE I WAS WATCHING THE TFC; THE FO CHANGED THE SPD IN HIS CDU. HE WAS STILL IN RTE 1; AND SOMEHOW IN THE PROCESS OF MAKING THE SPD CHANGE; HE ACTIVATED RWY 24R. RIGHT AT CIVET; HE NOTICED THE ERROR; I RE-ACTIVATED RWY 25R AND HAD US DIRECT TO THE NEXT FIX. WE MAY HAVE GOTTEN 1 MI N OF THE TRACK. ATC CALLED TO VERIFY THAT WE WERE FOR THE S COMPLEX. I ACKNOWLEDGED AFFIRMATIVE AND STATED THAT WE WERE CORRECTING. THE REST OF THE APCH AND LNDG WERE NORMAL (EXCEPT FOR THE DIFFICULTY OF MAKING THE DSCNT). THE REASON FOR THE SAFETY RPT IS TWOFOLD. 1) TO RPT DEV. 2) TO RPT EVEN WITH A PROPER BRIEFING AND PREPARATION; ATC IS ASKING TOO MUCH OF INBOUND FLTS. WE NEEDED 3 SETS OF EYES TO WATCH FOR THE TFC; MAKE CHANGES TO THE FMC AND ALSO FLY THE ACFT WITH NUMEROUS CHANGES. IF THIS WERE AN ISOLATED INCIDENT; IT WOULD BE DIFFERENT; BUT UNFORTUNATELY THIS IS MORE THE NORM; NOT THE EXCEPTION.