J.R.
I sing the clean up song (from Barney) and have my two year old help- we also race to see who can clean up the fastest. If that doesn't work then I would grab a big black garbage bag and tell them that they have five minutes then you start picking up- then follow through and start picking up- if they let you take them away- put them away and keep this up for a week- as their toys disappear they may rethink cleaning up- then once they start cleaning up- give back the toys. The other thought I had was- how cluttered is the room- is it overwhelming to clean it all up- perhaps have them clean up before its such a huge job that they cant get it all done in five minutes or less.