I saw this tip here maybe a year ago. Tried it on one car, which still has a good exhaust pipe. I think this goes a long way towards making a 240 exhaust system last longer.

The weak point on these cars seems to be the under-axle pipe, right where it goes into the muffler. I've had several disintegrate right at that point, when the rest of the pipe was quite sound. Someone finally figured out, and posted here, that the frequent failure there is due to a stress fracture of the tubing.

Usual failure mode:
You (or someone) attach the axle pipe first and then attach the muffler to it, and all seems well. Then you hang the tailpipe on the muffler. That tailpipe weight on the muffler rotates it along it's cylindrical axis, putting stress on the axle pipe where it enters the muffler. It's easy to "see" in your mind's eye if you've worked on these a bit. The axle pipe can't flex with the rotation since it has essentially a right-angle bend and is fixed to the first muffler. So there's this constant twisting stress on the pipe where it goes into the muffler, made worse by constant vibration and corrosive exhaust. Pretty soon it rusts out right there.

Easy Solution:
Set up the axle pipe and attach to first muffler (mid-muffler).
Hang rear muffler with axle pipe inserted but totally loose.
Attach tailpipe to rear muffler and medium-tighten the clamp, so that tailpipe aims horizontal when muffler is hanging freely.
Now, with muffler and tailpipe balanced, go to the muffler-axle pipe joint and clamp it. Muffler will already be in it's non-stressed position.
Now go back to tailpipe, adjust slightly to level if needed, and tighten the clamp.

My last tip - avoid short trips in the car. On short trips the exhaust system never heats up enough to vaporize the water and other corrosive stuff. Exhaust system life is probably measured by the number of cold shutdowns (shutdown when pipe is not hot) more than anything else. Next most significant item would likely be the number of starts.
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DAMHIK: Don't Ask Me How I Know - - - Sven: '89 245. 951 ECU, open-front airbox, E-fan, IPD sways, E-Codes, amber front corner reflectors. 500 mi/week commute. '89 245 #2 (wifemobile). '90 244 (spare, runs).