This tips may work on other linux but I only test it on Debian Squeeze :
mx1:/var/log# free -m
total used free shared buffers cached
Mem: 1961 1532 429 0 207 701
-/+ buffers/cache: 623 1338
Swap: 513 0 512
mx1:/var/log# sync && echo 3 > /proc/sys/vm/drop_caches
mx1:/var/log# free -m
total used free shared buffers cached
Mem: 1961 296 1665 0 0 23
-/+ buffers/cache: 272 1689
Swap: 513 0 512