My advice:

- Never sell. Ever. (Until retirement). [1] The natural extension of this is... always buy. Buy now. Buy.

- Keep a pool of cash on the side (whatever you can afford) to be ready to capitalize on any "fire sale" of stocks. This should be about 10% of your portfolio as a very liquid non-volatile asset (cash).

[1] Obviously life hits you hard sometimes and you HAVE to sell to cover unexpected bills. I am obviously not suggesting in those situations that you hold your stock to the detriment of your healthy, of a family members health, etc.