Here's one that seems so obvious, but until someone suggested it the other day I'm not sure it would have ever occured to me!
When diving in chilly water, or on a not-so-warm day, take along a thermos of warm water to flood your wetsuit with before diving. (be careful to make sure it's warm, not HOT, -- we don't want anyone to scald themselves!
On the dive boats in California we'd do this in the shower -- fill up with nice WARM water, then don gear and go in....very effective!
This would be the 'take-along' solution for diving where there's no warm shower nearby.
warmth on a chilly day 
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