You will probably get several different answers to this question, but this is what I would do (have done). Flush the knife out with running water to get a lot of the grit out. Sometimes a toothpick, cotton swab, modified chopstick will help get a little more out. Then I would apply a healthy dose of White Lightning knife lube to each blades pivot point and open and close them several times. For whatever reason this stuff tends to migrate crud out of the knife as you do so. White Lightning is not a great lube for all knives, but it works great on SAKs.
Good luck,
Paul