The best location of the sensor is generally on the muscle belly, far from tendon origins and innervation zones. The sensor should be aligned along the direction of the muscle fibers, which typically run longitudinally in a muscle.
For routine applications, the skin surface should be wiped with an alcohol swab to remove oil and debris. For more extreme conditions, such as in case of dry skin, repeatedly apply and peel hypo-allergenic tape to the skin sensor site to remove the outer layer of dead skin.
Firmly apply the sensor to the skin to ensure no movement occurs between the sensor contacts and the skin.
If your EMG signal contains excessive line interference, check your environment for electrical devices that could introduce noise in your recording. Stay as far as possible from external noise sources.