Visa renewal
Important news concerning visa renewals.... 
When applying for your original visa, keep a copy of all the information you used.
If you have a copy of your original application forms, when it comes to renewing your visa, you have all the answers ready at hand.
Basically the same documents are required for the renewal of your visa as the original application.
The main difference with the renewal application is that you can apply for a visa renewal inside the country. That means that you can apply at the foreign police office that is relevant to where you live.
It is not possible to cover what documents are required in an article such as this. There are too many answers for all the varying requirements.
The basics are:
Reason to stay.
Proof of funds.
Accommodation.
If you have all the above, everything should be relatively smooth.
Where to renew your Czech visa?
In most cases, you will need to renew your visa at a foreign police office. Which office depends on where you live.
Further information on the Prague foreign police offices.