There are two approaches, change how you're driving, and modify the car.
The latter is to simply block off the front grill, improving aero drag and reducing the amount of cold air to the radiator and engine compartment. Look for work by Recumpence and ptjones, confirming engine temp is OK, and mileage rises.
Others have hit on the driving changes to some extent. As I just noted to someone else, there are several stages to warmup.
Stage 1: cold - ICE on continuously, turns off when warm, not hot
Stage 2: warming
warming 1 - EV only available with heat OFF
warming 1a - ICE will come back on and stay on
warming 2 - EV limited to 1 bar with heat ON
Stage 3: hot- full EV available and ICE turns on/off with throttle
I drive uphill for most of my first mile in the morning. I run defrost max until I hit the main road; turn heat off and I can EV. I take this as far as I can, and get a long second warmup run as a result. After the second ICE warmup run, I can EV with the heat ON, but only use 1 bar, perfect for a long flat run. From here, I play with heat and throttle but retain the 1-bar limit until I'm a good 5-10 miles from home...
And that's on a 25F day. Haven't had any lower, yet...