You are getting good advice. #1 transmission killer is heat. Certainly Install an external trans cooler if your car doesn't have one, and, even if your car does have a factory cooler, consider increasing the cooling capacity for your trans by replacing it with a bigger one. I recommend the same one willwren uses. I installed a smaller size from this company, and saw great improvements, but now I wish had bought the same one Bill recommends - this one:
Consider installing a drilled 180F thermostat to further lower drivetrain temperatures. With the engine and trans running cooler, everything will last longer.
Drop pan, clean, change filter and refill - will change about half the fluid. I do not recommend trans flush because of the possibility of major trans damage when not performed properly. Safer and cheaper to drop the pan yourself. If the trans is shifting OK now, I would not add anything but Dexron III. I have had some success with additives in weak transmissions that were on their way out, but I would not use them in a good trans.
Also good advice on the easy driving. I have babied many a car with weak parts and gotten many more miles out of 'em. You cannot routinely do hard acceleration and expect any transmission, strong or weak, to last as long as if you drove it gently. Driving hard and fast reduces the life of bearings, axles, brakes, suspension parts, tires, fuel economy, and driving privileges.
If your engine is an L36 (non-supercharged) read this topic:
Do this repair work BEFORE the LIM gaskets or the UIM fails and damages your engine.
This, and frequent oil changes (not by Jiffy-lube) ought to keep your '98 running for a long, long time.