This is what I've learned so far...
1) Buy low, sell high - the only stock advice (it may have been some catchphrase in the '80's... and the simplest, thanks again Occam's Razor) I ever heard growing up (not that I knew anyone with extra cash to invest in anything other than pooling our pennies for a 6-pack of Bud, until I found out post-mortem that my father, unbeknownst to me, had taken himself from a depression-era kid working the potato fields along the Eastern Shore to a damn-near millionaire in one lifetime (Dad I'm learning-curve my a** off for us, aiming to make you proud and your grandchildren wealthy))
2) Have at least a little something at every risk level (which ETFs make totally possible) to cover your behind during those highs & lows... which leads me to...
3) Fluctuations in the field - no worries, those lows will give you a chance to get back in and buy more of the good stuff (which again ETFs make totally possible)
Good Luck & Good Fortune!