Overall an appropriate amount of liverock, not over-stocking, proper flow, proper feeding habits, along with Protien skimming and regular water changes will go along way.
That being said you need to provide more information if you want any meaningful help.
What size tank are we talking about?
How long has this tank been established?
What is in the tank as far as live stock?
What are you using for flow and circulation?
How large of water changes are you doing?
What are you using for lighting?
Do you test your water?
What are you test results for everything that you test for?
I always have stuff dying bc my water won't stay right.
What stuff is dying? and What makes you think your water is not right?