Being on guard is a precautionary measure we take to ensure safety and security. To express the same sentiment, we can say "be vigilant", "be watchful", "be alert", or "be wary". These synonyms can be used interchangeably depending on the context or situation. "Be vigilant" highlights the need for careful observation and attention; "be watchful" emphasizes the need for vigilance over a period of time; "be alert" suggests a heightened sense of awareness and readiness to act; and "be wary" expresses caution and suspicion. Having a diverse vocabulary of synonyms for "be on guard" can help us convey our thoughts and ideas more effectively and accurately.