Draw energy into yourself, focus on it becoming flame inside of you. It's a similar idea to how you focus energy before casting a spell, only instead of casting it you pull it into yourself
Alternatively, if you identify with specific elements, try these:
Visualize pouring warm, steaming water into yourself and feeling it fill you, starting from your toes and all the way up to your head.
For air, I know A woman who likes to imagine a warm, southerly wind flowing through her in gusts and eddies.
Earth is more commonly used where I'm from; many practitioners I've worked with like to imagine that they're laying or surrounded by sand, bleached and heated by the Sun.
It is possible that techniques like the above have a psychological effect in warming up your body, but they should be paired with external measures to keep your body warm. I don't know what you mean by glowing inside, however.
What helped me psychologically was envisioning lava pouring over my body and recalling the heat of fire - this helped me bear cold mornings walking to the bus when I was little. Later on, my father taught me that if I'm cold (and already bundled up), I should wrap my arms around my core instead of trying to rub my arms as my core is already warmer and that would spread to my arms. In my experience, he was correct).
I found out that some Tibetan monks can control the temperature of their skin through meditation. Specifically, they have been shown to be able to raise their skin temperature, measured from their toes and fingers, as much as 17 degrees Fahrenheit, while their core temperature stays normal.there are also Indians that can lower their metabolism by 64 percent. For perspective on how remarkable this is, when you sleep your metabolism drops only 10-15 percent.
Well there are certainly stories that say such things as the ones kingphilios mentions happen, but I know of no verifiable scientific stories that prove that such claims are actually true.
I agree with Lark, there are a lot of Tibetan monk stories out there. I've seen some pretty crazy stuff out there, and even I'd be skeptical about some of it.
