Spend some $$. There is no such thing as good and cheap in a refractometer. Most of the ones sold by the HBS are too cheap. The handheld ones that are good come out of the wine industry suppliers and run upwards of $80 to $100.
I had one I paid about $40 for and it drifted like crazy and rarely matched a good old glass hydrometer. I was given a Milwaukee digital model (they run around $100 I think) and it is a thing of beauty. Rock solid. calibration is a snap....though it rarely moves, and it's very fun to use. I buy sterile, individually packaged disposable pipettes, they're like $5 for 100, so tracking gravity on fermenting worts is actually a pleasure rather than being a PITA.