This is the method my husband uses in the pharmacy...he says it works every time...
Tip the jar upside down while holding the jar in your hand and tapping lightly on the bottom of the jar with the butt of your opposite hand 4-5 times. Continue to let the jar set upside down on the counter for 4-5 minutes. This will allow the fluid inside the jar to disolve the dried residue of the fluid that has caused the lid to stick. Then use a dampened wash cloth or a rubber lid remover to unscrew the lid.