Taking the lid off your slow cooker, even just to give it a quick stir, drastically reduces the heat and extends the cooking time. Only lift the lid if instructed to do so in the recipe.
No Preheating Required
A slow cooker should never be preheated. Simply put the stoneware container in the slow cooker base, cover and set the desired cooking temperature.
Slow Cooker Care
Do not add cold water to slow cooker immediately after use as this could cause it to crack. Either add warm soapy water or let the slow cooker reach room temperature before cleaning.