VFR PLT ENTERS IFR CONDITIONS.

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I AM A PVT PLT AND AM CURRENTLY IN IFR TRAINING. THE FLT WAS A VFR TRAINING FLT WITHOUT PAX. TOOK OFF FROM SAN CARLOS ARPT (SQL) AND STAYED IN THE PATTERN THERE FOR 3 TOUCH-AND-GOES. THE ATIS AT SQL INDICATED VFR CONDITIONS AT THE TIME. DEPARTED SQL FOR LVK FOR MORE PATTERN WORK. ARRIVED AT LVK; LANDED TO A FULL STOP AND TIED DOWN AT TRANSIENT PARKING TO USE THE FACILITIES. TOOK OFF AND DID 4-5 TOUCH-AND-GOES. DEPARTED FOR SQL FOR MORE PATTERN WORK. AFTER LEAVING THE LVK CLASS D AIRSPACE; TUNED TO SQL ATIS. THE ATIS AT SQL INDICATED VFR CONDITIONS. AT THAT TIME I WAS AT 3000 FT OVER SUNOL GOLF COURSE AND BEGAN MY DSCNT INTO SQL WITH THE INTENTION OF CONTACTING SQL TWR AT COYOTE HILLS. ATIS HAD RPTED 7 MI VISIBILITY AND OVER 3000 FT SCATTERED. I COULD FAINTLY SEE COYOTE HILLS FROM SUNOL AND CONCLUDED THAT IT WAS OVER 7 MI. I; THEREFORE; ALSO CONCLUDED THAT THE RPTED CEILING WAS ACCURATE. APPARENTLY; THE CEILING HAD DETERIORATED SINCE THE LAST ATIS UPDATE WHICH WAS ABOUT 25 MINS OLD. I PROCEEDED WITH MY DSCNT TO COYOTE HILLS. WHEN I WAS OVER FREMONT; AT 1300 FT AND ABOUT 5 MI FROM COYOTE HILLS; THE VISIBILITY DETERIORATED. COYOTE HILLS BECAME FAINTER. I MADE A DECISION TO TURN AROUND AND GO BACK TO LIVERMORE. SINCE I HAD DSNDED OVER HILLS TO FREMONT; I FELT THAT I NEEDED TO DO A 180 DEG CLBOUT TO THE L. THAT WAS MY MISTAKE. I SHOULD HAVE MADE A LEVEL 180 DEG TURN TO KEEP MY VISUAL GND REFS; AND CLBED OVER THE TERRAIN AS I NEARED IT. DURING THAT CLBOUT; I WENT INTO THE CLOUDS AT ABOUT 1600 FT. I BECAME DISORIENTED WITH RESPECT TO GEOGRAPHY; TOPOGRAPHY; AND LOCATION. MY ORIENTATION WITH RESPECT TO THE ACFT ATTITUDE AND ALT WAS FINE -- BASED SOLELY ON INSTS. I KNEW THAT THERE WERE 4200 FT PEAKS IN THE VICINITY; SO I CLBED TO 6000 FT. HAVING NEVER FLOWN IN THE CLOUDS; I BECAME CONCERNED TO THE POINT WHERE I WAS TOTALLY FIXED ON THE INSTS TO KEEP A SAFE ATTITUDE AND ALT; SO I DID NOT THINK OF OTHER STRATEGIES TO DSND BELOW THE CEILING. I CLBED EVENTUALLY TO 8500 FT AND CIRCLED; AND HAD STILL NOT REACHED THE CLOUD TOPS. I THEN BECAME CONCERNED ABOUT THE C152'S CAPABILITIES AT HIGHER ALTS; SO I SQUAWKED 7700 AND MADE A 'MAYDAY' CALL ON 121.5. EVENTUALLY; STOCKTON CTL CONTACTED ME AND GAVE ME RADAR VECTORS TO A VISUAL LEVEL; ABOUT 5500 FT; NEAR TRACY. I LANDED AT TRACY AND CALLED TRACON; AS DIRECTED BY THE STOCKTON ATC; AND LET THEM KNOW THAT I WAS DOWN SAFELY. AFTER THE FACT; I THOUGHT THAT THE BEST ESCAPE STRATEGY WOULD HAVE BEEN TO USE THE MODESTO OR STOCKTON VOR AS A REF; FLOWN 'TO' IT; CIRCLED AND DSNDED TO A VISUAL ALT.