This method works equally well with plastic and metal models. I'm not sure if it would work on the Humbrol type enamel paints though, but definately sorts out acrylic paints. I've also used Nitro-Mors for stripping before, Dettol works just as well and is not carcinogenic - Bonus! :-)
You will need:
1) A bottle of Dettol - see the picture below, it's available from any supermarket
2) An old toothbrush
3) A glass jar with screw on lid
4) Rubber gloves (otherwise your hands will smell for a while)
Pour enough Dettol into a clean jar to cover the model. Stick the model in and pop the lid back on. Leave the model at least 24 hours!
You'll know that the model is ready to remove because the paint goes all baggy and starts to separate from the miniature. See below for an example:
Put your gloves on and fish the model out of the jar. Dip the toothbrush in the jar and use it to scrub the paint from the model. The paint should come off very easily, but you may need to keep dipping the toothbrush in the Dettol to keep it lubricated.
It's important that you don't wash the model in water at this stage otherwise the paint will turn into a sticky mess! Use some more Dettol to rinse the paint from the model
Once the paint is away from the model, it can be rinsed under running water, and dried in a bit of kitchen towel. It may be a bit smelly, but otherwise completely ready for painting!
There you go! Hopefully this was of use :-)