First make sure that the problem is not in the way you are cooling your pie. To keep the pastry from getting soggy, make sure you cool the pie on a wire rack. This will keep the pie elevated so that air can circulate around the bottom. I know it's tempting to cut into a pie right from the oven. But don't. If the pie is cut while hot, you break the bottom crust and it will soak up the hot fruit juices and get soggy.
If that is not the problem then try these ideas.
Bake the pie on a lower rack in your oven. This will concentrate heat on the bottom of the pie and help the crust crisp.
Use a glass pie plate. Glass pie plates give the flakiest results and produce a more evenly browned crust.
Partially pre bake your pie crust before adding the filling.
Brush the bottom crust with corn syrup before filling. This works like a seal and will keep the crust from absorbing the filling.
If the recipe calls for adding the filling while it's hot, make sure you add it hot, don't let it cool before you add it to the crust.
Hope this helps and happy baking.