Mxconns is an X11 proxy that prompts you for individual connections,
and lets you optionally disable "dangerous" X requests.  Useful
when logging into a potentially untrusted machine via ssh.

Quick start:
in your .xsession:
export XDISPLAY=`mxconns -fork -hunt -verbose -icf "c" -debug "al"`
later...
DISPLAY=$XDISPLAY ssh -f <untrusted host> xterm

for more info, read the man page.