A software tester must have intent of showing the product 
is not working as specfied.
Software tester have the basic attitude of showing the 
presence of errors.
he must have perspective of customers i.e he has to use the 
system as he is the client of the system.
he has to strive for the quality.

pls inform if i am wrong
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Software Tester must has these qualities---
1)He/she must observe the problem from both the side say 
user and programmer.
2)Must has good under standing  with other team members .
3)Able to understand programmers view.
4)Once start testing , do not put it remain.
5)First test requirements of the user. 
6)Before start testing first analysis the project like ; 
technology using in project, all the flow etc??

Know the technology:the technology in which the application 
is developed so that u can concentrate on the weakness of 
that technology .
Explorer: tester should be a bit creative and have an 
attitude to take risk will help him in finding bugs

Troubleshoot: Troubleshooting and figuring out why 
something deosn't work helps testers be confident and clear 
in communicationg the defects to the developers.

Objective and accurate: testers should be very objective 
and know what they report.

A GOOD tester has the following qualities
1. Test to break attitude
2. His view as Customer.
3. Should be tactful & diplomatic.
4. Strong desire for quality.
5. Excellent communication skills.
6. Should have some development experience.

Ability to write effective test cases.