How to write an effective use case

1) Use case is a second document which is basically written by SME or Business development executive
but some times in a small companies sr test engineer or manager will write due to limited resource never send a techie because he is having a limited domain knowledge once it will be wrong it will effect the whole project so usually those people send to get requirement who is expert in domain it should be in a story mode or it will be written in a points depends on sme understanding. basic of story mode they will create a use case use case is generally a positive scenerio it can not cover negative test cases so when ever u r creating a test case behalf of use case or requirement docs always think about the positives and negative scenario it is a best practice to get a good result.

2)  once u develop the test case better send a docs to the team members they can review before placed in a center for available anyone the best practice is to update the test case at every release.
3)If u r working in a scrum frame work it si difficult to update test plan at daily basis but u had to write new test case and send it for review than u can observe that what changes r required. sometimes u can not able to understand the requirements if people will review your docs and gave the guys to proper response and feedback.