Using ‘so’ to intensify an adjective is common in speech and informal writing (I was so tired after the exam, or we waited in line for so long this morning) but avoid it in formal or professional writing. Instead of ‘so tired,’ ‘exhausted’ or ‘beat’ might convey the same meaning in a richer way. Instead of ‘so long’, try ‘eons’ or ‘endlessly.’
‘So’ is meant to be partnered with another clause, and in these instances it is acceptable (I was so tired after the exam that I feel asleep on the bus, or we waited in line for so long this morning that we missed lunch). When you see a so, ask yourself so what? A second clause should be there to answer the question.
Sometimes the second clause can be implied, but avoid this in formal writing. I’ve never been so thrilled [as I am at this moment]!