When it comes to staying vigilant and alert, there are a multitude of synonyms for the phrase "on guard." Depending on the context, you could use phrases such as "watchful," "vigilant," "attentive," or "alert" to convey a heightened sense of awareness or readiness. Other possibilities might include "prepared," "ready for action," "armed and ready," "standing by," or "on the lookout." Ultimately, the exact phrasing you choose will depend on the situation at hand, but by using synonyms and variations on the theme of "on guard," you can convey a sense of readiness and determination that is sure to impress.