My daughter who is now 13 and diagnosed not a saltwaster has played soccer, softball and basketball.  We have ALWAYS found that because the exercise was more intense than she was accustomed to doing, we had to increase her meds.  We used to give more when we got home if she needed it.  Then we found that if we gave an extra 1/2 dose BEFORE a game she was much better at the end of the game than if we didn't.  We got by with an extra half dose BEFORE the game instead of an extra whole dose AFTER.  My theory was that if we knew she would require the additional med and if we could give less before and prevent her from being ill afterward, it made more sense to do it that way.  I did not do this before practices, but her coaches have always known that she has her limits - especially in hot weather - and have gone a little easy on her at practice. The problem usually occurred at a game where they needed her to play more than she would have at practice. Her endo was aware of how we dosed her before a game and agreed.  We also make sure she is well -hydrated and would give a salty snack before.We kept Gatorade available for her on the sidelines.  Her endo said that although she is not a typical salt-waster, she certainly would lose more than a normal amount of salt during vigorous activity and that is why the extra meds and salty snack were important.  As she has gotten older and sports are more competitive, we have elected to drop the softball and basketball (she hasn't played soccer since she was about 8).  We found coaches are more demanding and less understanding as the kids get older.  Kristin now rides horses and that is her sport of choice. 

I know there is concern about oversuppressing with the extra meds, but what we did was an extra 1/2 dose once or twice a week - when we knew she would need it.  She is now 13 yrs 6 mos old, 5'1-1/4" tall, and her bone age right on target at 13-1/2.  i am not advocating that everyone increase their child's meds for any activity, but offer it as a possible solution - one that has served my daughter well.