Set the table with placemats and cutlery where you prefer to have your Sims sit and eat. They will grab a serving then sit and finish their meal. Works like a charm. And there are several different place settings to choose from. Not sure if this option came in a certain pack or an update though. I've been using this trick for years. As for putting dishes in the dishwasher, Sims do seem to learn by being directed to clean up over time. However, I use a mod that prevents anyone from leaving the table and automatically clearing the dirty dishes. I rather enjoy having couples and families sitting for a bit and chatting after meals. And the dishwasher is rather useless as it can't wash more than one dish at a time. I use another mod called the 'Eco Dishwasher' from LittleMisSam that allows the dishwasher to be loaded ten times before it initiates the wash cycle.