Get Free Advice and Sell More
It will make the bond stronger, and they will give you important insight (and if you sell to them immediately after, you’ll get higher conversion rates.)
The primary technique is to ask for their advice on something. Asking for their advice (and not opinion or information) makes them cross over to your side. They are now your trusted advisor because they have committed to helping you. This can be done in a retail environment, website, email, etc. If they react you will gain insight from them, and you might want to make them a special offer after they reacted – they will be more open to it so your conversions should be higher.