Gas flow should be always across/ parallel to the sensor face, not onto/ perpendicular to the sensor face.
If you need to restrict flow, then place the restrictor upstream of the sensor.
Pumps create oscillatory pressure changes which can lead to high readings and should be upstream of the sensor if possible.
Sensor outlet should ideally exhaust to ambient to minimise pressure drops/ variations.
Include a particulate filter on your inlet if sampling in dirty environments. If measuring reactive gases such as NO2, H2S, O3, Cl2 then use filters constructed from fluorinated substrates if possible.
Avoid condensation in your gas train. The sensor temperature should be at least 5oC above the dewpoint temperature. Nafion dryer systems are frequently used to regulate the humidity of the sampled gas.
In-line gas filters should be considered to reduce cross-sensitivity effects.