Why 山? Easy: think of three mountain peaks. Historical forms of this character look even more like children’s drawings of mountains.
Combining 火 (fire) with 山, we get a word for volcano: 火山 (huǒshān). Now recall the word for sky, 天 (tiān). If we stick it in front of 山, we get 天山 (Tiānshān), “Mountain of Heaven”, the name of a great mountain range in Central Asia. That’s how I remember the pronunciation of 山. For Russian speakers, or indeed anybody who is familiar with Cyrillic alphabet, there is an additional mnemonic: 山 looks very much like Ш, the first letter in the Шань.