One of the biggest challenges singing live is monitoring the sound, hearing what everybody watching hears. That way we can adjust instruments and volumes so that the "mix" is right.
There are two ways of doing this, both with pros and cons;
The first is the traditional wedge speakers, whilst they feedback the "entire sound" to the band, I have found some hiccups with these. This is because it can be hard to distinguish my vocals from example the lead guitar, also they can cause feedback which anyone who's experienced it will associate with being extremely irritating!
The other challenge is that even the best singers out there aren't strong enough to out do an amplified guitar cranked up, shredding, or even a drum kit out of the box. When I can’t hear myself over the band I can push my voice too hard.
The second option is the in ear monitoring system, much easier to transport that the wedge counterpart, (and I am a BIG fan of making kit easier to cart about!), but very much tailored to individuals and not the group. To get around this we run Adz, myself and Adz's guitar through the P.A which then plays back via the in ear monitors. Perfect on paper, the only challenge is that it means the band members wearing them will hear the band sound perfectly, but it doesn't necessarily reflect what everyone else hears.
For me, I am a fan of the in ears and what you will see before any gig is one of the band listening to us all play (Giles normally "gets" nominated) to get the mix right. And it can be quite entertaining to watch.
We're all still learning to use these gadgets, which is why when we play live I fiddle with my ears so much! but as the saying goes.....practice makes perfect.