Wow, a 23 year old A/C (that beats me by 2 years). A pro has to answer your question but I would think a tune-up includes a filter change, cleaning the evap coil, checking the condensor coil, checking system pressure, adding a little freon if needed, and checking the wiring. Hopefully you already know the price of your tune-up.

"Is there any preventive maintenance that could prevent a similar occurrence?"

- No, you are already on borrowed time. I would not have replaced an evap coil on a 23 year-old unit - I would have replaced the whole system, especially with the gain in efficiencies with a new 13 SEER minimum versus your old 7 SEER (a 45% increase in efficiency).

Not much you can do to prevent a leak in the indoor coil except reduce the amount of corrosives in the home. A lot of fumes from cleaners such as chlorox and ammonia may lead to premature corrosion on coils. I noticed years ago that the use of hair sprays and such during the winter would seem to accelerate rusting of the heat exchangers in some homes with standing pilot furnaces. Using chlorox in some drains is a no-no and will cause rapid rusting/corrosion. Vinegar is milder and less damaging in my opinion. Never add any acidic chemical directly into the pan of the coil.
There's quite a list of things checked during a "tuneup", which is an unusual name for it.
A lot of the checking is visual- looking for signs of corroded connections, burnt or wires getting overheated, mold in the plenum or duct (system not cooling properly), insulation on the refrigerant lines and drains, rusted pans, leaky ducts, vent pipes clear and installed correctly, drain is clean and clear, blowers and coils (and it's pan) clean, the outdoor coil (condensor) clean, motors oiled, freon psi normal per the proper charging method for that system, air split within range, thermostat accurate, return sealed, electrical breakers and wires the proper size and aren't running too warm, no buildup of leaves in the outdoor unit, no distributor tubes touching which can rub together and cause a future leak or wires rubbing and getting ready to wear thru and cause a short.
A quick inspection of the ducting, mainly at the plenum is a good idea. If molds are present at the plenum, it is usually a sign that the system is not operating properly (low on freon, low air split ect.) and throwing off moisture instead of removing it. If this persists, mold will develope throughout the ducting and all the way to the registers. The house will feel damp or warm even at comfort temperatures of 75-77 degrees. A check of the humidity and wet bulb temps if the system has been operating normally helps evaluate possible problems.
There's actually more and this seems like a whole day project, but actually an experienced tech does these visual inspections in a short time. Once any corrections are made (if any) the same tech should be the one to come back every year, as he knows your system.
I wash every outdoor coil, regardless if it looks clean or not, because I have been suprised before from the ones that look clean but are not.
Of course all of the controls should be checked, and any visual signs of oil on the freon lines or fittings checked to see if there is a leak and repaired.
The proccesses for checking heat pumps, cooling only and gas furnaces are a little different, but at the same time similar. If you have a gas furnace, I always give it a quick look for anything that is unsafe with it or anything applicable to it's proper and safe operation and advise the customer of any corrections needed or to have it looked at before winter use.
Some people think they don't get their monies worth when the tech isn't there for a long time, but a well installed and maintained system doesn't take as long to check over as compared to a system that has been neglected or installed poorly.
Prices are quite different in all areas and range from $50.00 to as much as $170.00. It's best to stay away (in my experience) from the $19.95 specials- no company can survive with that price and must sell something to make up the difference.
Phew! I feel like I just wrote a book! :)

Thorough listing daddo. I have washed some trane coils, xmas garland type and thought they were clean. Had to return and remove the fan and wash from the inside out also. Hard to know if a coil is really clean so I also never went by 'looks clean'. Wash it and know it's clean.

Don't know of anyone stupid enough to say $19.95 special but there are $49.95 ads. Beware of them. They make the $ on selling extras. Sometimes made up extras.

On a 1984 a/c, I would guess that the indoor coil is not the cleanest. I would have the tech find a way to look up inside and see the condition of the underside of the coil. A temp drop could tell, and a reduced air flow could be a clue but most don't really know how to use this info. Best to cut in and peek or remove a blower. Same with high efficiency furnaces. Secondary coils get overlooked.

Keep the filters clean, wash the outside unit at end of season, (cover top of it in winter), cut power and wash it again in springtime and look inside at the electrical connections (no mice chews), change the furnace filter and once temps hit upper 70's turn the unit on and let 'er run for an hour or so. Check/change the filter and if no odd noises, inside or out, you should be good to go once the temps hit the upper 80's. Change the filter. Check it on a reglar schedule. Keep the outside unit coil clean. Check the filter.

As for the a/c tuneup, I would run the unit after letting the house warm up to the outside temp if they are in the 80's or more. Set the tstat to 10 degress colder than that and close up the house and see how long it takes to cool the house. Chart the temps/time for reference.

You could shadow the tech but this is not always the best thing to do. You could ask or better to discuss his/her findings after he/she is done and ask your questions then. I never minded letting the homeowner know as much as they wanted to about their system as long as I could still bill for the time. Companies don't like techs to give out free 15 min informercials.

I just paid $129 for my AC check up here outside Philly. New AC (and high efficiency gas furnace) put in three years ago and sevice had been $99 until this summer. The tech who has been out each time to do the service said his boss has to raise the cost due to increased insurance and fuel costs.

I hang out with the tech...and he spent about 45 minutes with me doing the check up. He goes over the outside and inside stuff with me in pretty good detail.

The is a Goodman unit that has worked perfectly since the day installed.

I appreciate the thorough responses. I apologize for a mistake in my original post. The Air conditioner and furnace are 1994 models (not 1984). I am guessing that your responses remain applicable.