When markets hit new all-time highs and every asset class seems to be joining the party, it can be challenging to stick to your plan.There's always that innate human instinct to stick around for another 5% or 10% upside, even when your original objectives have been met and your rational brain is screaming to sell.There’s a trick to knowing when to sell and when to hold for more upside. Think of your stocks as you would your car or household appliances. They may work fine for a long time, even if mechanical ...