How to reduce scarring from cleft lip repair. Do it before birth?

Maybe in future. Presently achieving a proper anatomical repair while minimizing wound tension is the best method to diminish scarring. There is abundant research to show nearly scarless healing when surgery is performed prior to birth but current risks outweigh the benefits of this approach.
Experienced surgeon. Although doing surgery before birth, in the uterus, will prevent scarring, it is way too risky. The risk of causing death to the baby, or even the mother is very high, and is not justifiable for this problem. Go to an experienced cleft surgeon, and most of the time the repair will be excellent. Revisions can also make a big difference when necessary.
Yes. It has been shown that intrauterine surgery reduces the scar formation significantly. But, we are long way from routine use of this technology.