1. Consider putting a heatsheet-type bivy in your kit in place of the plastic bag. Heatsheet-type material reflects back a high percentage of your body heat and will act as a relector to trap a fire's heat.

2. I am going to assume you would not usually be far from your camp, or someplace with more resources, if you are encountering a hypothermia hazard equipped only with a garbage bag for spare clothing. RTB instead of staying in place and trying to build a fire.

3. Going with the scenario, however, perhaps you should build a large fire [or two] right away, then huddle naked at [or between] the fire[s] while drying clothes. Whether naked in a plastic bag without insulation or in wet clothes, you are losing body heat fast. You may not remain functional for long without replacing heat - get the heat source[s] going!

4. If there is no realistic hope for large fire[s] and all you have is the garbage bag, then consider stripping, getting into the garbage bag, wringing out your clothes, and hope you are wearing some high-tech stuff that absorbs little or no water so that it makes sense to put them back on.