Is it obligatory for me to say "Allahumma Barik" (O Allah, bless it) every time I see something I like, or is it sufficient to say it the first time? And will I be sinful if I don't say it on one of those occasions?

The Prophet, peace and blessings be upon him, commanded us to supplicate for blessings upon seeing something admirable in one's brother, and this supplication should be repeated whenever the admiration recurs. The ruling on not supplicating for blessings varies depending on the state of the beholder. If his admiration is intense and he fears afflicting his brother with the evil eye, then supplicating is obligatory for him. However, if that is not the case, then it is recommended for him to supplicate.