When my kids start to say no about picking up, I say let's do it together. That usually works. Sometimes I think they get overwhelm with the pick up or some feeling about something else come thru. It helps to not get into a battle. Give them a way to cooperate. Tell them it would feel good to do this together.

My son uses an activity table for legos or anythin with lots of small pieces. We try to not mix anything else onto the table when legos are out so it doesn't get too overwhelming. We us those big plastic box lids as trays for organizng Pokemon Cards, then stack them up under the activity table.

I Try to help my child learn to pick up one thing before he goes to the next. Some times I am fustrated with my kids about their toys and bedroom. I have to remember to ask myself: Am I a good example? Have I helped them learn this habit? They really want to please us but we need to help it along. A routine pickup time just before bed would help but I haven't fit that in yet.