How to Eliminate Virtually all Chance of Nuclear Weapons Being Used:

1. Get the nuclear powers, declared and otherwise, to agree to a verifiable disarmament schedule.  At a minimum, this means the following nations must cooperate:

Russia              ~10000 warheads

US                    ~6000 warheads

UK                    ~300 warheads

India                 ~300 warheads

Pakistan            ~150 warheads

Israel                ~150 warheads

France              ~100 warheads

China                ~50 warheads

North Korea      >20 warheads

South Korea      (Could probably deploy dozens within 6 months) (less if they are willing to piss people off)

Taiwan              (Could probably deploy dozens within 6 months) (less if they are willing to piss people off)

Japan               (Known/declared capability to deploy 80 within 6 months) (much less (5-10 days?) if they are willing to piss people off)

Iran                   (Suspected development program – might have a weapon by 2020—maybe 2012 if they are willing to risk war to do it.)

There are about 50 nations (including those above) with existing capabilities or even weapons-grade fissile material, but most have only small amounts and/or have no conceivable defense need for the weapons, so provoking the international community by withdrawing from the NPT would not be worth the weapons, and it would take long enough for them to make the weapons that an international response might have time to intervene.

2. Most of those countries will only agree to disarm conditional on US and Russian disarmament.  Therefore, the US and Russia must take the lead and eliminate their own massive advantage before other countries begin in earnest.
3. The US and Russia should disarm down to 300-500 warheads each, and place those warheads under Chinese-quality safeguards.
4. The 7+ member 100+ warhead club should disarm together down to the 20-50 range of China and North Korea.
5. All ten nuclear powers should physically dismantle remaining warheads, but continue to maintain parts in operable condition.
6. The existing IAEA should be reorganized into an international distributor/licensor of nuclear power facilities and fuel.  A fuel cycle that recycles fuel in such a way that no stage in the process produces a weapons-grade byproduct should be developed and become the world’s dominant fission energy technology. (several have been proposed that do this (which would be “Gen V” technology – currently most US reactors are “Gen III” or earlier – “Gen IV” reactors are coming online in several countries around the world: France, Japan, South Africa are the leaders, I think) These Gen V reactors would, like Gen IV, be physically incapable of meltdown, and consume 99% of the energy potential of the fuel – US Gen III reactors consume about 6%, which is why we have such a problem with nuclear waste.
7. The IAEA and member countries should systematically hunt down all fissile material – whether it is weapons-grade or not.  Users of the material should be monitored, and it should be accounted for at least as carefully as we account for money and drugs. (being far more dangerous than either!)
8. Sometime in the distant future, when the ten nations decide on their own that they no longer need to maintain their remaining 20-50 weapons, they should be recycled into fuel at an IAEA-controlled facility.

PS: While the stories about missing nukes make good TV news, there have been very few cases where a complete weapon was “lost” or stolen.  In all of the known cases, the weapon was recovered.  A much bigger question is whether or not all of the radiological devices in the world are accounted for, and the answer is no.  The USSR powered remote sensing posts (thousands of them!) with radiological batteries; Russia does not have complete records as to the whereabouts of these facilities, whether the batteries are still operable, and/or whether the radiation source has been removed.  These are not weapons-grade materials, but do include plutonium, so we are talking about high-level dirty bomb potential.  But by far the biggest supply of radioactive material is from hospitals in western countries – and the controls on those materials are minimal.