Record in a quiet environment
Use a directional microphone
Keep the microphone close to the sound source
Reduce background noise at the source
Apply a noise reduction filter
Use a high-pass filter to remove low-frequency hum
Use a low-pass filter to reduce high-frequency hiss
Apply spectral noise reduction
Use a noise gate
Use a de-esser for harsh sibilance
Remove clicks and pops with repair tools
Normalize and adjust levels after cleanup
Edit out silent sections with noise
Use audio restoration software
Record a noise profile and subtract it
Use equalization to reduce unwanted frequencies
Apply manual cleanup to isolated noise
Export in a lossless format when possible