"Best of Luck" (abbreviated as BOL by SMS generation) is perhaps one of the most used phrases in its category, specially so in India, I suspect. The phrase is way ahead of its formal and colloquial substitutes such as "Good Luck", "Achchha Karna", "Phood Ke Aana" etc. We use this phrase to support people about whom we care before their embarking on any form of multi-party competition (exams, interviews, sports match, etc.) where outcome is dependent on performance of a particular moment or day rather than on long-term steady play. Phrase explicitly acknowledges factor of luck in deciding outcome and hence is generally accompanied by prayer to that effect, vocal or silent.
I've used this phrase often too except that in recent years I almost always tend to use "Good Luck". Why? Let me try to explain. When we say "Best of Luck", we wish upon our recipient best part of their luck for that particular incident. Now, when recipient has used the best of luck for that incident, he will not be able to use that for any future incident. Of course, typically we say "Best of Luck" every time as if we are not seeking global optima but only pareto-optima, but frankly, how many times can we have best, if it really is the best?
We don't know what may befall upon on recipient later, and how serious that may be, and perhaps would be more needy of best of luck, rather than mere second best of luck. And since uncertain future misfortune is always at higher priority level than current exam or interview (except if event is someone's life's ambition beyond which he may not even care for death), it is useful to save best for future. In that sense, best of luck is like call option that has more value live than expired. Hence my reluctance to use the option and save it for later.