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There are several mnemonics that distinguish between the two similar geographical names/features. Here is one that also helps with the correct spellings:

" Stalactites point ceiling-downwards
while
stalagmites point ground-upwards."

Both features are columns of mineral matter found in underground caves and formed by calcium-carbonate deposits from water-drops. A stalactite is like a roof-icicle in appearance, with occasional rainwater trickling down its length. The rainwater contains calcium carbonate dissolved from limestone or chalk above, and as it trickles, the water partically evaporates to leave behind a small quantity of calcium carbonate - causing the stalactite to grow in length over thousands of years. A stalagmite is often formed on the cave floor immediately below it in the same way, as the raindrops fall to the floor. Due to the "splash" effect, it is usually shorter and thicker than the stalactite. When they eventually meet they form a single pillar.

John B. Harris of Anderson, Indiana USA tells us (12/99) that he learned an alternative mnemonic description in grade school many years ago:

"Stalactites Cling tight
while
stalagmites Grow mighty."

Acording to Marcus Lewis-Price (10/00), the best way to remember the most important bit is simply this:

"Tights come down!"