What is the best way to repair and prevent cracking grout in a shower stall?
This is combined problem. Firstly, your grouting material might not be suitable, but what is even bigger problem person who did ceramic tiles work did a poor job; distance between tiles is very, VERY big, and this is the reason why grouting has cracked. You should remove all of the old grouting with some appropriate tool; beware not to damage waterproofing layer because that would be the worst thing in the world. After removing grout clean thoroughly all the dents between tiles and then apply new grout. It wouldn't make sense for me to recommend you some specific grout because it varies from country to country (and continent to continent) but explain sails man or contractor your situation and ask for recommendation. So, what you would be telling them is that you need a grouting for bathroom floor; bathtub and that distance between tiles is bigger than it should be (measure the average) so you need grouting that will not crack.