Those of you who DIDN'T experience stomach issues during your IM - any tips?

Most GI issues racing can be traced to either over-consuming or bad pacing (racing harder than you trained).

Do your long bikes at a higher intensity than you will race, then be diligent about holding a lower intensity on race day (preferably managed by HR or power, not RPE, because race day adrenaline can throw your RPE off, especially for less experienced athletes).  Don't consume different fuel or at a faster rate during racing than you did during training.  The higher the intensity of the exercise, the more blood flow is diverted away from the organs to the muscles that need oxygen, so digestion doesn't work as well.  Also, the higher the intensity, the more glycogen (carbs) your body burns, and less fat.  The lower the intensity, the opposite is true (more fat, less glycogen).  This approach makes GI issues much less likely on race day.

Agree.

I use a custom mix by Infinite on the bike, and am very measured (by a timer) on my intake. I am always "steady" in my pacing so don't have any issues with over extending myself, either in training or racing.

I joke I have an iron stomach.  I could eat a tuna melt on the bike and be fine.  I am NOT a fan of infinat  I have several friends DNF and or be sick during long races because they were sent the wrong mix.  Too many variables.  My best advice is to keep it simple, have a back up plan, practice your nutrition and don't worry about being sick.  If you BELIEVE you are going to be sick you will be sick.  Sometimes its unavoidable but it makes me crazy to see people working themselves into a tizzy over this.  The mind is a powerful thing.  Have plan A, B and C.  What happens if you loose a bottle of the only nutrition you trained with and its not available on the course?  What if what you trained with suddenly becomes unpalatable?  what are the race conditions going to be?  What if its 20 degrees colder or warmer than what you train in? How will you handle that?  If you are married to ONE plan and are convinced you are going to be sick if you do anything BUT that you are setting yourself up for failure.  I also put a little roll of tums in my bento box and my tri top on the run.  I don't think I have ever used them after toeing the line at 8 Ironmans, but I have saved a few peoples races

Sadly, yes trial-and-error. 

I've done 9 IMs, with my 10th coming up two weeks from now in Arizona.  I'm a slow swimmer (bottom 20%), MOP cyclist, and top 25% runner.  I pride myself on running the entire run, walking on aid stations and staircases, that sort of thing. So my pacing and nutrition plan supports that race plan.

For my first three Ironman races (IMAZ 06, 07, 08) I had no stomach issues during the race. and then starting having them for the next few.  Eventually I isolated it to the CarboPro I was using.  I just can't handle CarboPro on IM race day despite training all season with it. I suspect it's the swim before the bike that is what's different than training days, although I didn't experiment on myself to find out. I just gave up on CarboPro a couple years ago and went gels/bars only, and voila no more stomach issues.

I bell-curve my nutrition needs rather than use a linear nutrition plan.  I eat more in miles 25-90 on the bike than I do at mile 1 or mile 110.  On the run I start hitting the Coke as soon as they have it, and grapes and orange slices. Generally I "live off the land" in the sense I eat what they hand out. I don't bring my my own food on the run, just my own gels (GU) on the bike. Salt stick tablets for hot races.

The last few years I've also been doing an hour swim Saturday morning before my long bike, and that makes a big difference to my nutrition needs on the bike.  I need more gels sooner or I'm low energy and lethargic.  In other words, if during training I determine my nutrition needs by cycling WITHOUT a preceding swim, then on race day suddenly put 2.4 miles before that 112, I will have an insufficient nutrition plan.

Good luck with IMAZ!  I've raced it four times, three times in the spring (yes, that long ago) and once in the fall a few years ago.

I've completed 11 IM races, mostly in the 12-13 hour timeframe, and had stomach issues on like #5 - #8.  Basically for me it turned out that what worked in training - CarboPro based "fuel bottles" - didn't work out in racing. After long introspection, I think that the swim made the difference. I trained on the bike with CarboPro (maltodextrine powder) based high calorie "fuel" bottles, with Gatorade and water.  And that was always fine.  But on race day, I'd tend to get sick at the 70-80 mile point and then suffer for the first 10 miles on the run. This happened time and again.

I think that the swim made a huge difference.  I rarely did a long swim before a long bike, but a couple times I did that during training that on purpose to see what would happen and yes, I'd get sick just like during those races.  So I gave up the high calorie CarboPro bottles and just used GU gels and Gatorade.  So much better!

My nutrition plan for IM #9, #10, and #11 has been a bell curve type of thing. After the swim, I chug some water and start the bike.  I ramp up my fueling (GUs and Gatorade) and max out at special needs with Pop Tarts (yes, cherry frosted Pop Tarts!).  Then I slow down my intake to let my GI tract get settled for the run.  I'm a really good aid station customer on the run.  GU and fruit and Gatorade early on, then I skip the Gatorade for Coca Cola after mile 5 or so. I'm a big fan of the orange slices and flat coke after mile 12 or so.