Tom, I found the biggest increase in heat I got was when I removed the plastic covers from the heat vents. Really increases the airflow by a big margin.
Also, I had a lot of drafts coming in from all the doors. Taking the time to adjust the striker plates for better sealing helped keep the cold out. Same with really making sure the dash vents are shut tight and have good seals on them.
Do you have a soft top or a hard top?
My oil line fix
My 100k stopped odometer fix
Brace for impact, I'm in a "modifying mood". Massive thread count inbound.