Arctic Silver doesn't go bad for a looooong time, but it does settle out if not used up quickly. You want a VERY clean small plastic tool (sort of toothpick-like) or you can clean a wood toothpick thoroughly with alcohol and let it dry - you don't want ANY foreign particles getting in your A-S. Then gently remove the plunger from the syringe (end cap removed of course) and gently stir the stuff back up well - you don't want air bubbles getting mixed in either. Unfortunately you will lose some every time you have to repeat this procedure - c'est la vie...
.bh.