Dump the cereals, M&Ms, nuts & pretzels in a large bowl. Melt the white chocolate in the microwave or in a double boiler. Melt very slowly, stirring occasionally, being careful not to burn the chocolate. Dump melted chocolate over the rest of the ingredients and fold over and over until you have well-coated hunks and chunks. Spread the whole mess out on parchment paper and set in a cool place until it sets up, then break it into pieces. Store in zip-top bags or air-tight containers.