I used to just use a lighter. You have to be careful or you will scorch the shrink tubing with a flame heater.
I bought a electric heat gun. It is an off brand and cost about $25. I use the thing all of the time. Best money I have spent in a good while. With the electric heat gun you will get a more uniform shrink and almost zero scorch issues.
I also use the gun to heat up tire patches. Hard plastic water line heat it up and it will slide right on your barbed fitting. Old radiator hose heat it up and it will soften up and yo can work with it much easier.