Review the mix before mastering
Listen for clipping, harsh frequencies, uncontrolled bass and effects that obscure important elements. Correct obvious issues in the project rather than trying to solve everything on the master bus.
Leave enough headroom for processing and avoid driving the mix bus simply to make the preview louder. There is no single peak value that guarantees a professional result.
Choose appropriate processing
Equalisation, compression, limiting and saturation can be useful, but each should solve an audible problem or achieve a deliberate character.
Compare processed and unprocessed versions at similar loudness. Excessive limiting can reduce transient impact and make the beat fatiguing.
Export and verify the file
Follow the delivery requirements of the platform or client. Check sample rate, bit depth and file format, then listen to the exported file from beginning to end.
Name files clearly and keep a final unprocessed mix alongside the master when appropriate. Do not assume that a successful export dialogue means the file is free from errors.