How do I prepare walls and/or trim for repainting (same color)? Do I wash with plain water? Soap and water?

Usually, washing them with dish soap and water, then rinsing, is good enough. If they're really greasy, you'll want to buy some TSP (trisodium phosphate). You can probably find it at stores like WalMart, but paint stores always have it. It's a powdered soap and it's super strong. It's been used for years by painters to clean greasy areas.How do I prepare walls and/or trim for repainting (same color)? Do I wash with plain water? Soap and water?
You are thinking ahead. Most people just paint over the old paint. You can either use just plain water, or soap and water. Be sure to let the walls completely dry before applying the paint.