A heater is a pretty drastic solution, although there are appropriate applications for heaters.
What minimum ambient temperature must your device operate at? Just about all electronic devices are rated down to 0C. -40C versions are usually available. If that's good enough, I would stop there and not bother with a heater.
How is this device powered? A heater will consume significant power. This is where thermal insulation helps. The better insulated it is, the less power will be required to maintain temperature in the cold. Of course, this works the reverse in hot conditions. How much does your device dissipate on its own? What maximum ambient temperature must it operate in? You don't want to add so much insulation that it gets too hot in the summer.
If you just want to keep condensation away, then perhaps the normal power used by the device is enough. Again, we need specs on your device.
If you do go with active heating, plain old resistors are the simplest. Place a few resistors around the board, and turn them on when the temperature gets below some setpoint. A thermistor on the board would be a good idea so that the heater is not turned on needlessly. Simple on/off control based on a thremistor threshold will oscillate a bit around the set point, but as long as the heaters aren't too overpowered that will be fine.