Always remember that there are humans on the other side of the app, software, thing you are using.
Feedback is critical - especially at the early stages
Why are things the way they are? It is easy to be critical, but chances are the person or team behind the choice spent a lot more time thinking about things than you did so think before you ask
Reproducible bugs only!
Something broken! Great - tell someone The most important thing you can do is include steps that show reproducible bugs
Help spread the word!
Did you like what you used? The team/person probably spent time adding in ways to spread the word so consider being someone that helps by telling a friend (best!) or spreading the word online.