I have posted the following last month for a specific design using a heart rate + pulse oximetry sensor from Maxim Integrated, but it should apply to other sensors that output periodic signals with noise:
http://www.instructables.com/id/Pulse-Oximeter-With-Much-Improved-Precision/
http://www.instructables.com/id/Pulse-Oximeter-With-Much-Improved-Precision/