Whichever method of spam filtering you use, you still need to check for false positives. But what I aim to do is to get the software to learn what I regard as spam. Systems like Thunderbird's adaptive filtering are designed to do this.
I very rarely delete spam manually - that is just too tedious. Instead, I quickly scan through the folder looking at message titles and subject lines, and mark the ones that I know are not spam. These are shifted back to my Inbox. Anything left in "Spam" gets deleted automatically after a set number of days.