A smaller pick ,like a Dunlop Jazz III works great for pinch Harmonics. The smaller size puts your thumb closer to the strings,you may even hit some by accident!! Personally, I find a thicker pick helps too,and the JazzIII's are around 1.35 mm thick. Also, heavier strings can help. I use an 0.11 set and the added mass seems to make harmonics sound bigger. Zakk Wylde uses a 0.60 on his low "E" for christsakes!! He gets harmonics like crazy. On the other hand, Billy Gibbons of ZZ Top is the king of pinch harmonics and he says he uses an 0.008 set. I've got four Tele's and a Strat, the best guitar I have for squeals is my 87 MIJ toploader Tele. I dunno if it's the Basswood body, the 0.11-0.49 strings,or what. But that guitar squeals the better than the others.
Once you get it down,move your pick along the string to get other harmonics. You can change the pitch of the harmonic by moving either farther away from,or closer to the bridge.