When I first start up my computer, free (and my GKrellM) reports I'm using 70-80MB RAM.

After a while, this creeps up to ~180MB, where it sits fairly constantly - Buffering and caching in action, I'm assuming..?

Except when I start doing a load of compiling (Putting together an LFS distro), when the memory usage drops back down to 80-odd.

So, two questions - why does compiling make reported memory usage drop?
Are there any simpler ways I can 'clear' the memory in the same way that compiling does, so I can tell exactly how much memory is ACTUALLY being used?

On clearing memory:
You should really let the operating system handle memory management.
Have you tried using the 'free' command to monitor memory usage?

Compiling theory:
perhaps when you compile the compiler is allocating memory that was used for the cache, and then when the memory is freed, since it is no long caching, you are actually seeing it as freed.

~Andrew

Yup - here's some typical output. The only thing different between the two was how long it had been since I compiled something. The same processes were running both times:

total used free shared buffers cached
Mem: 503 394 108 0 29 288
-/+ buffers/cache: 76 427

total used free shared buffers cached
Mem: 503 461 42 0 32 301
-/+ buffers/cache: 127 376