Sustain is how long a note rings before it dies. It comes from a mix of gain, compression, amp volume and technique — not just turning everything up. A little goes a long way before it turns to mush.
Add gain — but not too much. Distortion compresses the signal and feeds the note, extending sustain. Past a point it just adds noise without more length, so raise it until notes bloom, then stop.
Use a compressor. A compressor (clean or before your drive) evens out the signal and noticeably lengthens clean and edge-of-breakup sustain — the secret behind a lot of singing clean leads.
Turn the amp up. A cranked amp moving air lets the strings interact with the speaker and sustain (and eventually feed back) — bedroom volume kills sustain that a loud amp gives for free.
Favour the neck pickup. The neck pickup is warmer and rings longer than the bright bridge pickup, which is why so many sustained lead tones use it.
Work the technique. Fretting-hand vibrato keeps a note alive, and controlled feedback (facing the amp) turns sustain into infinite sustain. Fresh strings and good setup help too.
💡 Compression + neck pickup + a touch more gain gets you 80% of the way. The last bit is amp volume and feedback — physics you cannot fake at whisper levels.