Well, alot of people have good advice about this. Make sure to have good flow, parameters, etc. Suction some of your substrate with each water change, clean filters, etc. Getting rid of excess nutrients helps. Others will reply with more...
Use a siphon to clean a portion of your substrate regularly. This will remove organic debris and uneaten food. Also, be sure not to overfeed your fish. Only feed as much food as they can consume in a few minutes.
Be sure your aquarium has good water movement. A powerhead or wavemaker will help increase water movement.
Perform regular water changes and routinely replace mechanical filter media.
Maintain proper marine pH between 8.1 and 8.4 with buffers or with kalkwasser. A high pH helps discourage aggressive growth of Red Slime Algae.
Supplement existing filtration with phosphate-removing chemical filter media.
Use a reverse osmosis system for quality source water. This will help prevent buildup of excess minerals in aquarium water from the get-go.