There are various synonyms that can be used to replace the phrase "build up your strength." Some possible alternatives include "strengthen," "fortify," "boost," "enhance," "amplify," "augment," and "develop." Other options could include "improve," "advance," "progress," "cultivate," "nurture," and "foster." Depending on the context and intended meaning, different synonyms may be more appropriate. For example, "strengthen" might be used to focus on physical strength, while "develop" could refer to personal growth and character. Ultimately, using a range of synonyms can help to keep language fresh and engaging, while conveying the desired message effectively.