How do i keep my score high?
To keep a good SCHUFA score (above 90%), those are the few recommended things:

Don’t open too many bank accounts in different banks and limit the amount of credit cards you own
Don’t switch bank account too often
If possible, try to keep healthy reserve in the accounts you own
Pay your bills on time
Check your SCHUFA-score once a year and correct any mistakes you might see
Are there any other institutions rating consumers in Germany?
As it turns out, there are several. SCHUFA remains the most important and influential one but other ones can hold data about you. That is a bit scary i know, and quite unexpected from a country so strict about data privacy. You can find out about them and request a record from each via this website for free.

2 more important things
The SCHUFA will hold information about you long after you leave Germany and at a European level too. If you plan to leave unpaid bills behind and then come back to Germany, you might have to think twice before doing it. It could become harsh back fire in the near future.

This system makes that your personal wealth, your personal property or yearly income doesn’t have any influence on your record ! You could be the richest man in the world but still have the worst record if you didn’t pay your bills. Just bear that in mind.