Give a Demo
Pretend a very interested audience is asking you really good questions and spend an hour or two answering those questions by demonstrating the system. I find more bugs this way than any other way.
Someone else will think differently than you do, click differently, and look at a system differently. The less you can tell this person - your second set of eyes - the better. You don't want to accidentally lead them down the trail through the system you always follow if you don't have to in order to explain the system to them.
Change the Environment
Use a Test Outline
A brief, high-level outline helps ensure you visit all the functionality you need to. Too much detail and you will focus on the test at the expense of finding issues. Also maintaining a detailed test takes a lot of time. Too little detail and you will miss critical areas of the system.
Review Your Work... Again
I recently saw a great entrepreneur video where they said, "The difference between Good and Great is 10 minutes." Which they explained meant that when you write your all-done email, don't send it. Get a coffee, walk around the block, then re-read your all-done email and try your system one more time from the perspective of the person you are sending it to. In software, that difference might be 2 hours instead of 10 minutes, but you get the idea.