Something that I read a long time ago, and I think makes sense - Avoid filter words as much as possible. 

Filter words are basically things like "she saw" or "she heard" or "she felt", etc. What they do is to create an additional layer between the story and the reader, and stops the reader from being fully immersed. 

E.g. 

She saw him approach her and felt fear. (she is telling me what happened)

vs

He moved towards her, and with every step he took, the trembling in her hands became worse. 

Ok, sentence two may not be perfect, but it's probably a lot more vivid than sentence one. 

There may be times when you want to filter something through your protagonist's POV, and that's alright, but generally you should want to reduce the barriers between your story and your reader. Pull the reader into your world, don't build walls to keep them out.