Note: Linux is NOT "eating" your RAM! Please take a look at Geirha's excellent answer below to understand why...

After the above note, if you still feel "cleaning" the cache could help, you can certainly try: it's a one-liner from the terminal:

sync && echo 3 | sudo tee /proc/sys/vm/drop_caches

There's no easy way to disable the cache, but you can achieve the same effect by cleaning it as often as every minute, if you want to:

• Make it a cron-job

  
  



• Press Alt-F2, type gksudo gedit /etc/crontab, and add this line near the bottom:

  
  

  */15 * * * * root sync && echo 3 > /proc/sys/vm/drop_caches

  
  



• This cleans every 15 minutes. You can set to 1 or 5 minutes if you really want to by changing the first parameter to * or */5 instead of */15

  
  

One liner to know REAL free RAM, excepting cache

Geirha's answer explains the details, but in short, you get the number of free megabytes with:

free -m | sed -n -e '3p' | grep -Po "d+$"

which on my 2GB command-line server returns an extremely health 1835.