Having spent some time in outbound sales (after tech burn-out), the most important aspect (as many comments say) is "relationships". The best training for that is to go out and make them. We had sales training every single day, so it's really not something you can pick up a book or go to a weekend class and walk away being effective. That said, books and classes are a good way to find your footing.

Never Eat Alone - Keith Ferrazzi (networking & relationship building)

Never Sit in the Lobby - Glenn Poulos (sales & relationships)

Getting to Yes - Roger Fisher (negotiation, particularly "principled negotiation")

The Joy of Selling - Steve Chandler

The Psychology of Selling - Brian Tracy

In one of our quarterly division training, our office manager gave us Dale Carnegie How to Win Friends and Influence People and were told if we learned nothing else, to study that book.

It's been over a decade since my sales time, but the 2 sales techniques I haven't forgotten are: "selling ins't telling" and "feel, felt, found". As you can imagine, they are about relating to people, not giving technical/spec speeches.

It's something you have to practice everyday, make sales a part of your job title -- not simply something you do on top of running the company. An integrated layer no different than other software maintenance task, except the maintenance is the relationships with people you want to sell to.

For any other tech types that may someday find they need sales skills, I highly recommend actual job experience in outbound sales (with a company that provides frequent sales training). It was a massive culture shock that gave me the professional people and relationship skills I struggled with.