Sing on key.
One of the first things I'd mention is make sure to take advantage of octaves. You can sing up an octave or down, it doesn't matter. You probably already know that, but it doesn't seem to register with some people. If you don't know the song, you can bullshit it with the vocals on. I prefer to have the vocals off, but some of these songs I don't know or barely know, so I fake it...