OS X is pretty stable these days. And with memory so cheap, it’s easy to leave programs up for a long time (as well as the OS). But sometimes you need to send some of those programs away.
For example, I was looking at Activity Monitor, which I happened to have sorted by Real Memory at the time. To my surprise, Safari – which had only been up a day or so – was using nearly 2 GB or RAM for “Safari Web Content” (as opposed to around 200 MB for the program itself). This was with 2 windows and maybe 6 tabs open – I think maybe one had a Flash movie in it. Closing up all those tabs and windows barely made a dent in the memory usage – it was sitting at 1.9 GB with all those windows closed. Only by quitting Safari altogether did I get the usage down (on restarting it was at a little over 200 MB).
Another offender was iTunes. It too was using 2 GB of RAM, and it wasn’t playing anything – no music, no videos, and the store wasn’t up. It was sitting on a list of songs (so not even cover graphics). Yet it was using up 2 GB of RAM. Sending it away and restarting brought it down to a more reasonable 170.5 MB.
So it pays not to keep things up for too long. Fortunately, I have 16 GB of RAM on this system, but it doesn’t take much (edit a large photo or movie, for instance) for a lot of that RAM to be swallowed up quickly. And if the system has to start swapping to disk, you’ll start to see the beach ball quite often.
Bust out Activity Monitor once in a while, not just when your system is slow or acting funny, but when it’s running well, so you can see what normal/average/good numbers look like. It will give you a jump on trying to figure out what’s wrong. And don’t be afraid to send programs away with the Quit command.