One of the simplest and easiest things that you can do to immediately improve voice quality and prevent vocal fatigue is to drink water and lots of it! Aim for at least 2 litres a day.
EXERCISE THE VOICE
The voice box contains muscles and ligaments which need to be exercised in order to create a stronger and more powerful voice. Practice little and often, gradually building up your vocal stamina; this is far more effective than a 2 hour session once a week.
BE AWARE OF THE EFFECT OF ALLERGIES
If you are a sufferer of either asthma or rhinitis then your voice may well suffer too as the allergic response contributing to these conditions has been found to occur throughout mucosa in both the upper and lower airway.
AVOID SPICY FOOD & DAIRY
For some people this may not be a problem, but for others spicy foods can cause acid reflux. Dairy can also be mucus forming causing the vocal folds to work less efficiently.
REST A TIRED VOICE
If the voice sounds hoarse or husky then try to rest it completely. Trying to sing on a cold can cause damage to the vocal folds and will delay recovery even further. If voice problems persist after 2 weeks then consult your doctor.
BE AWARE OF THE EFFECTS OF MEDICATIONS
Many medications such as anti-histamines, anti-depressants, decongestants (remedies for nose/sinus congestion) and diuretics will dry out the vocal folds. If you're unable to find a natural alternative compensate with increased water intake. Commonly prescribed steroid inhalers actually cause vocal fold inflammation and hoarseness.