If I have a strained voice I find 2 things help: turn the gain up on your mic so u can sing softer and not strain, and make sure your monitor is REALLY loud and clear so u can hear yourself really well for the same reason. In ear monitors are the best option if u can get used to them. (And with keyboardists you can save a bundle because you don't need a wireless version.)
If your mic is hot and you can hear yourself well, you can sing for days.