As already mentioned, one of the key ingredients to good heater performance is stopping cold air from coming in. The best heat system in the world can't do much if you have air coming in that is colder than an AC system provides in the summer.
It is counter intuitive, but the first places to focus on for leaks is wherever the air is going out. This is usually in low pressure areas because of fuselage shape (canopy sides, along roll bar on a tip-up, etc.) If air is being sucked out because of low pressure, there will always be cold air coming in somewhere else to equalize pressure.
Then focus on pathways that air can come in. Don't worry about sealing everything because if you want vent and heat air to enter easily, you have to have a path for air to exit. I prefer to leave the corrugated ends of the baggage bulkhead open and seal everything else. This provides a flow path from the front of the cockpit to the back, maximizing heater and vent performance.
Strips of Kleenex can be used to find areas where air is moving out or in.
Once you have sealed up the cockpit, then work can be done to improve the temp delta of the heating system, but the biggest benefits are usually seen with time spent stopping the cold air from coming in.
Any opinions expressed in this message are my own and not necessarily those of my employer.
RV-6A (aka "Junkyard Special ")