Quoting an interview with a Somali Pirate in Wired. (Tricorne tip:  Snarkmarket.)

1. Bargaining Power of Pirates

Often we know about a ship’s cargo, owners and port of origin before we even board it. That way we can price our demands based on its load. For those with very valuable cargo on board then we contact the media and publicize the capture and put pressure on the companies to negotiate for its release.

2. Bargaining Power of Foreign Negotiators

Armed men are expensive as are the laborers, accountants, cooks and khat suppliers on land. During long negotiations our men get tired and we need to rotate them out three times a week. Add to that the risk from navies attacking us and we can be convinced to lower our demands.

3. Intensity of Competitive Rivavlry

The key to our success is that we are willing to die, and the crews are not.

4. The Value of Hostages

Hostages — especially Westerners — are our only assets, so we try our best to avoid killing them.  It only comes to that if they refuse to contact the ship’s owners or agencies.  Or if they attack us and we need to defend ourselves.

5. The Threat of the Navy

Whenever we reach an agreement for the ransom, we send out wrong information to mislead the Navy about our exact location. We don’t want them to know where our land base is so that our guys on the ship can manage a safe escape. We have to make sure that the coast is clear of any navy ships before we leave. That said, there is no guarantee that we won’t be shot or arrested, but this has only happened once when the French Navy captured some of our back up people after the pirates left the Le Ponnant.