A MOSFET will enter its saturation mode and leave the 'linear region' when an appropriate voltage difference between the gate and source has been achieved. For most N-Channel MOSFETS, this is between 8 and 15 volts. 12v Vgs is what I see most drivers providing MOSFETS these days.
To "guarantee the MOSFET to run in saturation mode even in any extreme environment/factor,"
You must ensure that Vgs never drops below the 'fully-on' range. Refer to your FET datasheet.
This is analogous to the way BJTs work. If you want a maximum output current, you must provide a minimum input current. With MOSFETS, if you want minimum output resistance and 'saturation,' where the FET effectively functions as a current source, you must provide a minimum voltage between the gate and source.
Here is a graph from the 06N03LA:
You want your MOSFET to have the same resistance regardless of the current passing through. According to the graph, if Vgs is 10v, the MOSFET will behave like a current source, generating minimal heat in the transistor itself.