"Mantenha seus pensamentos positivos, pois estes, tornar-se-ão suas palavras. Mantenha suas palavras positivas, pois estas, tornar-se-ão suas atitudes. Mantenha suas atitudes positivas, pois estas, tornar-se-ão seus hábitos. Mantenha seus hábitos positivos, pois estes, tornar-se-ão seus valores. Mantenha seus valores positivos, pois estes tornar-se-ão seu destino."
"Keep your thoughts positive because your thoughts become your words. Keep your words positive because your words become your behaviors. Keep your behaviors positive because your behaviors become your habits. Keep your habits positive because your habits become your values. Keep your values positive because your values become your destiny."
I agree with Goncin.
'Keep your thoughts positive' (or, if you prefer, 'make sure your thoughts are positive') sounds ok and has a meaning that is subtly different from 'keep positive thoughts'.