Welcome to the Village Laura! Thin woods tend to warp because of uneven moisture absorption. You didn't say where you get your wood. If you get it from a local source, when you bring it home, don't lay it flat on your bench. Lean it so air can circulate all the way around the wood, or use sticker or dowels and stack the wood with weights on top till the wood acclimates to your shop.
If you get you wood online, leave it wrapped in the packaging it came in to acclimate to your shop, then stack and sticker.
this will ensure your wood will sta flat.
good luck!