[vermaden]

install ccache port or add a package:

# cd /usr/ports/devel/ccache && make install clean
# pkg_add -r ccache

add the following to /etc/make.conf:

CC=/usr/local/libexec/ccache/world-cc
CXX=/usr/local/libexec/ccache/world-c++

add the following to /.cshrc:

# set ccache varibles
setenv PATH /usr/local/libexec/ccache:$PATH
setenv CCACHE_PATH /usr/bin:/usr/local/bin
setenv CCACHE_DIR /var/tmp/ccache
setenv CCACHE_LOGFILE /var/log/ccache.log

# set ccache temp size to 512MB (default 1GB)
if ( -x /usr/local/bin/ccache ) then
  /usr/local/bin/ccache -M 512m > /dev/null
endif

logout and login again before using ccache because when You dont ccache will use /root/.ccache dir instead of /var/tmp/ccache.

ccache can be shared between several computers the same as ports tree for example, check man ccache for more info.

ADDED 2007.2.15:
example ccache stats from my box.

% ccache -s
cache directory                     /var/tmp/ccache
cache hit                          18562
cache miss                        102820
called for link                     9824
multiple source files                 75
compile failed                      1610
preprocessor error                  1446
not a C/C++ file                    3747
autoconf compile/link              16982
unsupported compiler option          511
no input file                       6698
files in cache                     49631
cache size                         464.3 Mbytes
max cache size                     512.0 Mbytes

ADDED 2007.2.16:
comparasion of buildworld times with and without ccache:

# without ccache
make -j1 buildworld  4148.38s user 937.02s system 97% cpu 1:27:00.40 total

# with ccache
make -j1 buildworld  1043.30s user 703.76s system 88% cpu 32:50.02 total

If you face a problem with /root/.ccache being used instead of /var/tmp/ccache then you may solve it that way:

% cd /home/${USER}
% rm -rf .ccache
% ln -s /var/tmp/ccache .ccache
% ls -l .ccache
lrwxr-xr-x  1 ${USER}  ${USER}  15 Dec 19 15:20 .ccache -> /var/tmp/ccache
% ccache -s
cache directory                     /var/tmp/ccache
cache hit                         165292
cache miss                        327142
called for link                    38002
multiple source files                216
compile failed                      5182
preprocessor error                  4934
couldn't find the compiler             1
not a C/C++ file                   26249
autoconf compile/link              52665
unsupported compiler option         1379
no input file                      23289
files in cache                     79438
cache size                         530.2 Mbytes
max cache size                     512.0 Mbytes
% sudo ccache -s
cache directory                     /home/vermaden/.ccache
cache hit                         165292
cache miss                        327142
called for link                    38002
multiple source files                216
compile failed                      5182
preprocessor error                  4934
couldn't find the compiler             1
not a C/C++ file                   26249
autoconf compile/link              52665
unsupported compiler option         1379
no input file                      23289
files in cache                     79438
cache size                         530.2 Mbytes
max cache size                     512.0 Mbytes