A well-matched cross-promotion can add 200–800 subscribers in 24 hours. A badly-matched one can burn the relationship for life. The difference is in the matching.
Match on reader, not on size. A 4K channel with the right audience out-converts a 40K channel with the wrong one — every time.
Match on tone. If your channel is calm and the partner is loud, the subscribers you gain will churn within a week. Tone fit predicts retention.
Trade fairly. Send the partner more value than you take. The best cross-promo relationships compound — five trades over five months will out-grow fifty one-off shots.
Write the post for them, not for you. Their reader doesn't know you yet. Give them a reason to care in the first line.
Measure, debrief, repeat. Every cross-promo gets a one-page review: subs gained, share rate, churn at day 7. The data tells you who to trade with again.