It is critical to test your applications and contracts thoroughly on the testnet as part of your road to the mainnet. Testing will help you understand how to create stable and robust applications using the Flow development stack.
All contracts should include test coverage for all contract functions. Make sure you've accounted for success and failure cases appropriately.
Once you deployed your application to the testnet, you should record how your application handles non-trivial amounts of traffic to ensure there are no issues.
You should also get familiar with the Cadence anti-patterns to avoid avoid problematic or unintended behavior.