My wife and I are big fans of Love and Logic. They are based here in Colorado and their general principle is that you as a parent need to take care of yourself. One of the ways you do that is to make sure your kids problems stay their problems.

In the case of toys the Love and Logic folks suggest that the best way to solve this problem is to ask your child once and only once for them to pick their toys

The way to ask is something like "Are you going to pick up your toys and keep them or am I?" When they don't pick up the toys by the end of the day (or your specified time) then you pick them up and keep them for at least a day.

With any luck the kid(s) will throw a fit when they can't have theory toys back the next day. Then you laugh at them during the fit and ask them to throw a bigger better more professional fit  The laughing usually throws them.

If your kid(s) act like mine they may not care the first couple of times.

However, one of the best techniques we've come across is to buy them a big bright new toy they love and take that for at least a day every time they forget to pick up.  My kids have so many toys that they didn't always care.  If you keep it, eventually the lesson will be learned.

Once my got to be about 4 the lesson was learned very quickly.  Before that it took more effort.  Part of our problem was that I didn't want to take the toys away from the oldest just because the youngest didn't pick up.  Then they share the toys and the lesson isn't learned as quickly for the youngest.

Keeping the toys took some time to sink in with our 3 year old. It certainly limited the number of toys I had to pick up the next day.

Your kid(s) will probably argue with you about how it wasn't fair or how they won't do it again.  The perfect response is "I love you too much to argue".  Just keep repeating it with a smile.  Your kid(s) will learn fast not to argue because you won't get stressed and won't give in.

It really is amazing how well it works.  Love and Logic is phenominally effective.

I would be honored to be an affiliate of Love and Logic but they don't seem to have an affiliate program.  Here's a link if you are interested - Love and Logic Parenting.