I have been getting this for years. I used to put gold bond medicated powder on it, but it really didn't help that much. About 3 years ago I read that this is caused by yeast. Since then I have been using monistat cream under my breasts when the skin is red and irritated. I also use tinactin fungus cream when it is really bad. That usually clears it up pretty fast. When I had a mammogram a few years ago I had the rash, and the nurse told me to take one of my husbands undershirts and cut it into strips about 2" wide and long enough to go all the way under my breasts and use that when I walk or am going to be outside and sweating. This helps a lot also, (when I remember to use the strips). It's a constant problem for me during the summer, but it's a lot better than before I started doing the cream and the undershirt strips.
One thing that can help is to make sure that you wear bras that are not too tight and that are made of cotton. Cotton breathes best and, that area in particular tends to collect most of your sweat in the summer so, it can get rashy. I would also suggest that, if you know you won't be going out in public for the rest of the day, to remove your bra while you are at home. That will help let your ladies breath and may help thwart the rash in addition to the monastat and your hubby's shirt. One last thing is that if you know you are going to be doing anything active that may make you sweat, wear a bra without underwire. I know it is less supportive but, that kind of constiction is going to make you sweat more there than if you have a looser fitting, underwire less bra.