Get Both Hands Involved!

Are you left-handed or right-handed? Place the pestle in your dominant hand. Next, take your other hand and cover the mortar, leaving a gap between your thumb and forefinger to pound through. This prevents things from flying out during the first few strokes. Once the paste gets going, you need only do this if you are getting splattered.
Rehydrated ingredients such as reconstituted dried chilies can become a soggy mess in the mortar. Be sure to squeeze out as much water as possible before adding them. Excess water will make for splashy pounding and result in more spoilage-prone curry paste (higher water content enables bacterial growth).