When all of us were studying at school or college (and even now) we have periods of 45 minutes or 60 minutes in length. Have you ever thought why this is limited to 60 minutes or 45 minutes? The answer is simple students will loose concentration after 45 minutes. Testers are like students and they will loose concentration after a certain amount of time. When you are testing an application make sure that you take a break after 45 minutes and then come back with a fresh mind to resume testing. When you are writing test cases ensure that the test cases does not contain too many steps. If your test case is lengthy it may take hours or days to execute. I have seen testers writing test cases with 400 or 500 steps which may take more than a day to execute.
The tester may start the execution of the test cases may be with Build 1 and by the time he reaches 50% of the execution you may have Build 2 from the development. You may be forced to start the test case execution from Step 1 again if the feature or requirements under test underwent a change in Build 2. It will be a good practice to limit the number of steps in your test case to 20 or 25 steps so that you can complete the test case execution in less than an hour's time. It may be not be possible to limit the number of steps in test cases for example when you are testing a complete end to end flow of an application. So try to execute test cases or perform adhoc testing in sessions of shorter duration and find as many defects as possible.