It's easy to do, but it took me a little while to find out how...
Covert the "Inactive" memory used by my Mac (as displayed in Activity Monitor) to "Free" memory.
First you might want to know what the difference between the two is. This is explained clearly by Apple support here: http://support.apple.com/kb/HT1342
And yes we know that the Mac has an incredibly smart memory management system, and it's wonderful that Macs are able to distinguish between completely unused memory ("Free") and memory that was used earlier but that hasn't been marked as "Free" just in case we want to use it for the same app again ("Inactive").
Unfortunately my experience is that if you have next to no "Free" memory left, and tons of "Inactive" memory, everything you start up feels sluggish.
So, to solve that problem manually, all you need to do is to flick open a terminal, type purge, and hit enter.
It may take a little while depending on how much "Inactive" memory you have, but after that things will open up a tad quicker.
Hope this helps :-)