Lighting! Lighting! Lighting! Single most important part of a good render in any software. Shit composition + good lighting can look okay. Good composition + shit lighting looks like shit.
Composition. Learn classical illustration and photography composition. Rule of thirds. Golden ratio. All that stuff.
Expressions. A big reason why a lot of Daz renders look like shit is because the people don't know how to create realistic expressions. They either leave everything with the blank face default, or they over crank the expression morphs and make it look hideous. Another mistake is expression symmetry. People don't have symmetrical facial expressions. Just like having a dominant hand people have dominant sides of their face.
Assets. If you don't know how to make decent models then get good ones. Also be prepared to fix the texture maps. A lot of models even commercial ones come with pretty shitty texture maps. Get a program that lets you paint on the model and touch of the texture maps to make them look better.