The most important thing to know, when sailing an 18 for the first time is to know if your rudder system is old style or new style. The change was in 84 or 86 I don't remember exactly.
You can tell the difference by looking at the locking mechanism in the lower rudder casting. If there is a black plastic cam like on a 16 then you have the new system and they will kick up when they contact the bottom or etc.
If this cam is not plastic, but silver metal (usually only unannodized metal in the casting) then your rudders will not kick up when they make contact with the bottom or etc.
If you have the old style and your rudders hit something hard and try to kick up the rudder or casting will usually break. Either way the rudders can be very expensive and the castings can be expensive and hard to find as Hobie doesn't make them anymore. If you can't find a replacement casting then the upgrade to the new style is around $300.
If you have the old style simply come into the beach with your rudders up or part way up. The other option, which I usually did, was to pull one rudder up and hold the tiller crossbar up with the other rudder completley down so I had good steerage but as soon as the rudder hit it would come up.