At very high nausea and intensity ratings, the excitement ratings drop dramatically. If you've got really high nausea/intensity, it probably means you are taking corners too fast or not banking them, or taking loops and other special track elements too fast.
Good rules of thumb:
-Start small. Don't build your lift hills so high. You can do a surprising amount of stuff off a lift hill only 12-14 units higher than your start height.
-The faster you're going, the longer turns you need. Some coasters are banked 50 degrees or 60 degrees (for example, the mine train coaster) vs the usual 30 degrees. These can take turns a little faster. This is indicated in the coaster type selection screen.
-Bank all your turns that are taken at over 10 mph.
-Good ways to build up excitement on a ride are to use a sine-wave type design. Basically, lift hill up about 12-20 units (Let's say 16 above ground level), then go down to ground level, then back up to 13 or 14 above ground level, then down to ground level again, then back up to 12 or 13, etc etc. Then when you need to turn, do it at one of the peaks so that the coaster isn't going so fast, until you slowly work your way down to the starting height of the station.
-Make the train criss-cross over itself when possible. This gives the illusion of almost going to hit, raising excitement.