Openshift cartridge goes idle after 24 hours of inactivity.
Activity is considered receiving a Get request in your application originated outside your cartridge (so pinging your app from your own cartridge doesn’t work).
You can use any free pinging service to ping your application after a specific interval of time(< 24hrs).
You may use Pingdom. I have found success using it. It provides a nice dashboard and graphs of response time as well. You will be notified if there is any issues connecting to your app or if its down. You can manage your account with their mobile apps.
There are other free pinging services too. Feel free to Google and try out other services. Do comment if you find a good one, might be a great help for some :-).