Here's some tips- they work for me, and I do a mix of melodic singing and screaming as well. I taught myself how to sing, eventually got some training from a few coaches. Here's a list of stuff that helped me- and I'm NOT a good singer. I just stay in my range and write as smart as possible.
Get a keyboard. For melodic singing, your best bet is to map out every note you sing on the keys. This makes it infinitely easier to write harmonies and see where your sharp or flat.
Don't force your voice. When you push notes too hard you'll go flat and not even know.
In ear monitors. Honestly, THE single best thing I ever did that improved my singing was in-ear monitors. Some people have perfect pitch and can naturally sing in key, most of us can't. Once I could hear myself clearly my pitch and annunciations instantly got better. I'm sure a lot of you guys practice in small spaces with a LOUD band, this makes a huge difference. Just being able to practice with it made the shows I didn't use it sound better too.
And definitely do 10-15 minutes of warm-ups. If you go into a show/performance cold you can really hurt yourself. If anyone here plans on singing for a few years you have to take care of your voice.
And for the guys with cracking voices- your either singing way out of your range or your just dehydrated. I drink as much water the day of a show as possible, and try to limit caffeine. Dry vocal corRAB will crack, trust me. Embarrassing shit.
I don't know everything about singing, I'm still learning. Everyone is different. These are just things that helped me over the years. If you want to hear my stuff, check out www.myspace.com/rotation Thanks guys!