Warm up your voice before singing, particularly before singing extremely high notes. Jumping right in and straining to reach high notes without first warming up will strain your vocal chords. Warm up your vocal chords by singing progressively up and down the scale, using a vowel sound such 'ah' or 'oooh.'
Observe your breathing while you sing to see that you are not forcing your vocal chords to produce sounds they cannot. Breathe deeply but easily, sending breath in even amounts over your vocal chords to keep your chords in a healthy state.
Learn the difference between your head voice and your chest voice and how to transition between the two smoothly to create what some call the 'even voice.' Vocal chords often get damaged when people try to make this transition incorrectly. Your head voice is a tone that you produce when your vocal chords resonate in the throat, the chest voice is when they resonate in the chest or sternum. Head voice notes are light whereas chest voice notes vibrate slower and are therefore heavier. With some help from a good singing coach, you can learn how to blend these two registers successfully to strengthen your vocal chords.
Rest your voice. Don't speak or sing non-stop for long periods of time. Take regular breaks to rest your vocal chords; make sure you do so at least every couple of hours.