As a long term investor though thinking 'this too shall pass' is quite a good strategy which means at some point the high prices will fade and you will be left with your dividends and earnings which at current prices are not giving you much of a yield.

Also I've been following Jeremy Grantham who is now preaching that the end is nigh. He has a good track record https://youtu.be/RYfmRTyl56w

He recommends emerging markets and value stocks by the way where valuations are more reasonable.