So, the tip is one you often encounter, especially if you monitor your CPU memory with numerous applications or a few larger ones consuming lots of memory, like any of the photo editors, some music/video editors, etc. It's very easy to eat through both active and more so inactive memory to find it's slowing from paging in and out.
You can monitor the numbers with Apple's embedded Activity Monitor application or several good third-party ones, eg. xScan and iStat Menus. These are useful for understanding what the Mac is doing.
The Mac system memory counts free (readily available memory), wired (system use), active (user apps, files, etc.) and inactive (formerly used and reserved until needed). It's the last that an application uses when it's open and leaves when it's closed. This memory is reserved for the application(s) and files if they're reopened so they can load faster.
Occasionally though an application can eat lots of this space due to bad design, forgetfullness or necessity. Some apps just use space like your computer is a free memory buffet, some just forget to clear or release the space when it closes, and some just need it (large image, video or audio files), all of which OS-X keeps it for awhile.
There are many little free or cheap applications, especially on the Apple App Store, freeing up this inactive memory space. They vary is type, interface, features, etc., but they all do one thing to clear the inactive memory to only the system uses and open applications.
And that's the tip.
And why pray tell is this a computer truth? Well, it's because Apple gives you the same tool. All you have to do is this.
1. Open a terminal window. If you have never done this, go to the Utilities folder under application and open the "Terminal.app" (click the icon).
Note.--You can set up your terminal features in the .profile file but if you have never used terminal window before read about using it and this file first. Don't open it up if you don't know how to edit it.
2. In the terminal window type: purge
The computer will slow for a short while, including freezing all applications. This is because it's stopping everything while it finds and clears the unused inactive memory (remember some is used and reserved by active applications).
When the prompt returns, your inactive memory is available again. You can check the before and after by opening the Activity Monitor (Utilitie folder again) and click the "CPU" button where you'll see on the bottom the various memory (Free, Wired, Active, Inactive and Used).
Note.--Do not run this command when the backup is running. It uses inactive memory and clearing it will cause it to regenerate the space to finish the backup. It erases the space it used after it's done.
The normal Inactive memory generally runs between 10% to 15% and occasionally 15% to 20% of the Active memory (they're separate but the ratio is the key). Anything above 20% means a number of things, unused memory of an open application, a few closed applications or large files, Time Machine backup running, etc.
Anyway, that's the tip. A simple command, and beats an app all to whatever.