Add Beneficial Root Bacteria (Hydroguard, VooDoo Juice, Great White, or Rooters)
Lots of Bubbles (hydroponics)
No Light Leaks in the Reservoir (hydroponics)
Keep Grow Room Cool
Avoid Disturbing Roots, Especially Young Plants
Keep Everything Extremely Clean & Sterile

NOTE: Hydrogen Peroxide (H2O2) Is Only a Temporary Solution: Use alongside something else for more long-lasting prevention.