Usually, each week sales fall until they stop almost completely on the 5th week.
However, by inducing a state change, you can extend the sales period or tremendously boost sales which were otherwise stagnant.
In addition, if fan numbers are healthily increasing, songs marked as "very popular" (大人気) on the charts will slowly get sales due to word of mouth.
Refer to the Internal Data Guide for more information about this.
※Black portions of the text are taken from the manual, red portions are information taken from comments or the guide thread.
※Not recorded in the manual, so the conditions here are conjecture. Awaiting further information.
When you are on stage and get a PERFECT, occasionally stars will appear. If these build up, the chance of a state change will increase.
To improve the chances of stars appearing: