While a mission is active, go to the gun shop and purchase everyone upgrade for every available weapon and some grenade. After obtaining all of the upgrades, leave the gun shop, and use one of the grenades to kill yourself. This will cause the mission to fail and you will start the mission over. Once the mission restarts, you will have all of your money back from the purchases in the gun shop. Return to the gun store, and all the weapon upgrades will still have a checkmark. Repeat this process when new weapons become available.