That is a big question......
You can replace the stock heater with a larger and more efficient one. I can point you to some options.
You can add a second heater. Again lots of choices.
You can add a diesel fired coolant or air heater, both are a bit pricey but great to have.
On a 90, adding proper insulation and sealing is the best place to start and is adequate down to -30 or so with the stock heater.
Pissing people off on the "net" since 1983.
Land Rover. Turning owners into mechanics since 1948.