To increase furnace airflow, you'll want to make sure that your furnace is physically sized correctly for your home.
Also, don't assume that a furnace can handle running at full capacity all year round; furnace heating capacity will vary depending on the outdoor temperature and humidity levels. You may even need two furnace units to ensure proper year-round heating capacity.
If you think your furnace's airflow is inadequate or if your furnace unit is not operating at maximum efficiency, first check your furnace filter housing for clogs and clean it out as necessary. Check the furnace blower vents and gas line passageways for any blockages; remove them if necessary. If the furnace has an intake outside, ensure that the intake area isn't blocked by snow or ice. Finally, inspect your furnace for damage or other problems that might be impeding airflow.