I just went through the rudder system on my 18 last year. The original design was "sloppy" - or at least, it was prone to having lots of slop!
The bushings make a big difference. I also upgraded to the H-20 style adjuster, which removed more slop and makes trailering easier. Get new rudder pins too. Old aluminum pins wear down quickly. If the adjustment screw is frozen, drill it out and replace it. The only sources of slop that I haven't been able to eliminate so far comes from a slight mis-alignment between the upper and lower rudder castings, and from the slop where the tiller extension attaches to the yoke on the tiller crossbar. To combat the former, I added padding to the bottom and sides of the lower casting where the rudder rests against it (actually just used the loop side of some velcro strips) and bought Murray's rudder return kit (as recommended by srm, actually) which holds the rudders tight in place with a bungie cord.
http://www.w1dm.com/projects/HOTLINE/10 ... OTLINE.pdf