How can I improve my ventilation throughout my house?
For heating, the supply vents should be low on the walls, so that's a point in favor of winter performance.

You might be well served to have the system checked, possibly cleaned, and flow-balanced by a HVAC professional.

Note that a common misconception/error is that "opening up all vents to the maximum" is a good thing and reduces the workload of the fan - more air does move, but in point of fact, because more air is being moved, the fan is doing more work, and that takes more power - it is possible to overload the motor by "opening up" too many things fully (though this is usually controlled by less-accessible balancing dampers on the ducts, rather than louvers on the grilles in the rooms.)

The other major thing you can do before winter is to caulk, seal and insulate (or at least check the state of those things, and correct any deficiencies.)