There are many ways. Below are some of the options. You can mix and match to best meet your needs. I'm sure others will share what works best for them.
Heat source options:
Microwave, griddle, electric or gas burner (single or double burner), presto pots, I'm sure there are more...
Injection methods:
Hand pour then inject, blending block (inject both at once), some use foil between the mold, etc...
Molds:
The more you can inject at one time, the less you will have to reheat. Doing one mold at a time is very time consuming. Even if they are different molds, do as many as you can at one time.
I have found high temp silicone bowls hold heat better than pyrex. When not injecting if you are using a microwave put it back in there to help hold the heat.
If using pyrex, some set it on a hot plate to keep it warm - I have never done this but saw a video Frank made and it looks like it works well.
For the most part I am testing molds so I am normally shooting quite a few at once. If you are serious about shooting laminates you probably need to look into a blending block system (
http://www.caneycreekmolds.net/index.php?topic=116.0).
Next would be your heat source. You may have to experiment with the different options to find what you like best.
Lastly, try and do as many molds at once as you can.
Hopefully something in all this helps.
Jason