For best results, check the following:

Your breathing
Muscle Tension
Where your speaking voice lays in your whole range. It should neither be too high placed or too low.

Asserting as to why you are trying to lower or higher your pitch may help. Remember, it should feel physically and psychologically right. For singers who sing other people's songs, many a time you feel the need to sing at the original scale even if it is uncomfortable vocally for you. Don't do this. Sing at a scale that is comfortable for you. Not only will it sound much better but will also cut out any unnecessary tension due to incorrect scaling which is either too high or too low for you.

DON'T DRY UP
Watch your fluid intake.
Have warm drinks or plenty of water. 
Avoid milky drinks or drinks with milk products as these will produce excess phlegm in the throat which will in turn effect singing.
Avoid cold drinks.

Avoid too much alcohol. Although a lot of singers will not go on stage until they have had a drink to warm them up, the lack of inhibition it causes may lead you to misuse your voice. It is also an irritant and dehydrating.