Stuffing valuables under your mattress doesn't qualify as secure. Install a wall safe in a discreet location to keep your cash and jewellery locked away.
Most burglars are looking for a quick score and even if they find your safe, they will probably look for easier items to steal. Wall safes aren't expensive and you can pick them up in hardware stores.
Choose a location in a wardrobe, but one that isn't too difficult for you to access. Avoid placing it behind a painting, like in the movies, as tastes change and if you decide to take your art down you will be left with quite an eyesore.
Look for a wall safe less than 16" (40cm) wide (most are). This ensures that it fits between two wall studs. Depth is also important as there could be an obstruction from pipes or electrical cables.
Use a plaster knife to cut out the required space for your safe and after placing your safe into the wall, secure it to the studs using the required fasteners or galvanised screws. Check out the post below for more detailed step-by-step instructions.
Installing a Wall Safe [Concord Carpenter]