You need to make sure it does not have
sodium chloride in it. That is the ingredient that also strips hair color out of hair faster. So if your hair is dyed, it is good to find products that don't have sodium chloride - the color will last longer.
For a good over-the-counter choice, I would recommend the Loreal Eversleek / Everstrong / Everpure line of hair products. These do not contain sodium chloride, nor to they contain sulfates (which doesn't really matter for keratin smoothing, but some people feel they are harsh on the hair).
If you are willing to spend a bit more, this
sfree Moisture shampoo, sold by Healthy Hair Plus is really good. It is such a gentle, conditioning shampoo, but without sodium chloride or sulfates. I came across it when I was researching affordable shampoo to use after I had the keratin smoothing done last year.