A huge problem that amateur singers have is that they don't warm up properly.
Bottom line, they just don't know any beneficial vocal warm ups.
So if that's you, then you definitely need to change this bad habit.
An easy way to see how important it is, is to think about professional athletes. Most of the time, these guys and gals warm up for hours before a major event. This prevents injuries and helps them perform at the highest level.
The same goes for your singing voice. You can't just jump in a start singing without a warm up because your voice will just break down.
There's usually a link between how long you warm up for, and how long you can sing (to a certain extent).
Just like a basketball player needs to warm up their legs, you need to warm up your singing muscles.
This needs to be methodical.
Here Are Some Great Tips for a Warm Up Routine:
Pay attention to your body. If you're feeling stiff, get some exercise in. It can be as simple as going for a brisk walk.
If you feel refreshed already, start off with some humming. Start low and slowly raise your voice.
You can also do a variation on humming and use Zs.
Be patient. You can't jump out of bed and expect to have your best voice in a few minutes time. That just doesn't work for anyone.
Wake up early and drink warm fluids.
Remember, take it slow and let your voice warm up, don't force it.