Growing up in the U.S. my sense of temperature was naturally in Fahrenheit, which doesn't help when travelling to other countries. There is the mathematical way to pretty accurately convert between the two, but I've found it isn't quick for simple math. However, I've figured out how to switch from Celsius to Fahrenheit quickly when discussing weather. Start from 32 and add 18 degrees Fahrenheit for each 10 degrees Celsius. This allow you to quickly estimate the temperature by just using addition rather than division and multiplication. Since good baseline temperatures for Celsius are 0 (32), 10 (50), 20 (68), 30 (86), and 35 (95), it makes it simple to quickly figure it out.
To quickly estimate the reverse, I'd say subtract 32 and then divide by 2 (though this gets less accurate pretty quickly so don't use it for even boiling water or you'll be off by 10 degrees Celsius, which is a big deal)
Since the weather is actually a common topic to talk about, when you chat with a non-American, I hope these tactics help you out.
N. D. Moharo