I use CRED and it is helpful. Apart from that I have sheet with all due dates. 80% of my cards are on same due dates. Rest 20% are different to use it for payments which I want to pay later or for revolving purposes.
At the due dates, I sit down and track them manually by logging into all apps so that I don't miss them. Moreover, I keep eye on email, so I know what are due and what not.
PS: I own 16 credit cards.