AN A321 CREW; ON A VISUAL TO LGA; RECEIVED A GPWS 'SINK RATE' WARNING BELOW 500 FT; CONTINUED APCH TO LNDG.

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I WAS FLYING THE EXPRESSWAY VISUAL TO RWY 31 AT LGA. THIS WAS MY FIRST FLT IN APPROX 6 WKS. I AM AN INSTRUCTOR PLT AND WAS TEACHING THE PREVIOUS MONTH. I WAS VIGILANT AND FLYING 'BY THE BOOK' AND THOUGHT I WAS FLYING THE APCH PROPERLY. ON BASE LEG THE CAPT SAID WE LOOKED A 'TAD' HIGH AS I ROLLED ONTO FINAL. I LOOKED OUT HIS WINDSHIELD AND SAW THE RWY AND REALIZED WE WERE A LITTLE HIGH SO I DECIDED TO S-TURN SLIGHTLY TO THE R ON FINAL AND TRY TO INCREASE THE DSCNT RATE WHILE I WAS STILL FURTHER OUT TO CATCH THE VASI BY 500 FT. I THOUGHT I WAS DOING A REASONABLY GOOD JOB AND WAS RED OVER PMK AT 500 FT AGL. OUR SINK RATE WAS 1300 FPM BUT I WAS ALREADY ADJUSTING. MY TOUCHDOWN POINT LOOKED GOOD AND SPD WAS GOOD. VASI'S WERE SHOWING RED OVER WHITE SO I ELECTED TO CONTINUE THE APCH. AT THIS POINT WE GOT A GPWS 'SINK RATE.' I SHALLOWED OUT HIS DSCNT AND MADE A NICE LNDG AT 1000 FT DOWN THE RWY. ALTHOUGH THE LNDG WORKED OUT FINE; IN RETROSPECT I SHOULD'VE GONE AROUND AT 500 FT. IF MY DEPTH PERCEPTION OR ANY NUMBER OF OTHER FACTORS HAD BEEN SLIGHTLY OFF; INSTEAD OF A NICE LNDG I COULD'VE ENDED UP WITH A TAIL STRIKE.