For a software tester, writing Test Cases is one of the most frequent and major activity. Now here are some important and critical factors related to this major activity: Test Cases are prone to regular revision and update; Tses Cases are prone to distribution among the testers who will execute these; Test Cases are prone to dustering and batching; Test Cases have tendency of inter-dependence; Test Cases are prone to distribution among developers.

Keep the five factors in mind and here we talk about the tips:

1. Keep it simple but not too simple; make it complex but not too complex.

2. After documenting Test Cases, review once as Tester.

3. Bound as well as ease the testers.

4. Be a Contributor.

5. Never forget the end user.

Test Case Writing has solid impact on the whole testing phase. Therefore, the due attention must be paid during this activity.

Source: http://www.softwaretestinghelp.com/tips-for-writing-test-cases/