In order to minimize the amount of data transmitted over a X protocol channel, caching and compression systems can be used, so that X session applications are made more responsive.
LBX, Low Bandwidth X, was one of the first project of this kind, but it has been dropped in 1997. It ran as a proxy server (lbxproxy). It cached commonly used information compressed data transmission over the network link.
http://en.wikipedia.org/wiki/Low_Bandwidth_X
http://tldp.org/HOWTO/LBX.html
VNC introduced a revolution in this context, as it enables compressione by default. Even more simply, ssh can use compression, enabling this option in the ssh2_config file:
Compression     yes

Alternatively, compression can be enabled on the command line:

$ ssh -X +C -o CompressionLevel=9 username@remote

Very interesting projects are active in this context today:

- dxpc (Differential X Protocol Compressor)
http://www.vigor.nu/dxpc/
dxpc is an X protocol compressor designed to improve the speed of X11 applications over low-bandwidth links (such as dialup PPP connections)

- xmove is a computer program that allows the movement of X Window System applications between different displays and the persistence of X applications across X server restarts
http://en.wikipedia.org/wiki/Xmove

NX technology compression system is based on dxpc:
http://en.wikipedia.org/wiki/NX_technology

Here a comparison of remote desktop software can be found:
http://en.wikipedia.org/wiki/Comparison_of_remote_desktop_software