You can remember the meaning of 小心 with this easy mnemonic:
On a deserted mountain track a small (小) group of merchants were carefully heading towards the next town. Their hearts (心) sunk when bandits that roamed the area discovered them. They were saved by a kong fu master from Shaolin Temple.
Using mnemonics to remember the meaning of Chinese characters like can have an enormous impact on how long to learn Mandarin it takes.