1. Research into which MOS does the best for your forecast area and which one is best for each season. Do this by each day keeping a journal of each MOS prediction and the actual highs and lows.
2. With a combo average of GFS and NAM MOS, the model average will generally be within 5 degrees F of actual high or low over 90% of the time in the less than 24 hour time frame. To fine tune the model prediction, use your intuition, research into which MOS works best for that situation, and use forecasting experience for that location. Experience is SO CRITICAL since it is often mesoscale effects that influence highs and lows. Each forecasting region has its "quirks". Experience also plays a role when "unusual weather" occurs since the models have more difficulty when the weather is being extreme and anomalous.
3. Read info linked below on predicting highs and lows. A link is also listed on MOS limitations.
4. Link to MOS
NWS MOS PAGE: http://www.weather.gov/mdl/synop/products.php
Clicking the link titled, "One-Stop MOS" will show all the MOS for a city of interest.