Life can only be understood backwards, but it must be lived forwards ~ Soren Kierkegaard
In an ideal world, one would hold stocks when they are cheap and sell them when they become expensive, put the money in bank and wait until the stocks become cheap again only to buy back and repeat the process. Cool!!!!,
Except, there is a minor hitch ~ that there is vague possibility of the above happening in a dream and that too from a good night’s sleep.
Looking back, you can always say which stocks could have been sold or bought into. But do not let this fool you into thinking that in real time, you can just get in or get out so easily.
In the financial markets, hindsight is forever 20/20, but foresight is legally blind.
And so for most investors market timing is a practical and emotional impossibility.
OK, so how about the professionals….Can they time the markets any better than the average investor. Studies have shown plain index returns have time and again beaten the (alpha searchers) mutual fund managers/analysts or market timers over a period of time. (If you avoid  them, you will also save on the various expenses which the experts charge)
Being in the Financial Markets / Financial Planning services industry , I have seen innumerable people following analysts on TV Channels, subscribing to various newsletters, online sites for the holy grail tips & trying to time the markets. However they succeed far and few BUT fail time and again. The only people who consistently make money are the information providers and the tipsters. 
Remember the opposite of TIP is PIT and that is what many traders and market timers land after burning their hands. 
Try Market timing with not more than 5-10% of your portfolio and see the results for yourself. 
For Serious investing, Timing is Nothing!!!!  Who the hell said, .. “Successful Investing is ~ Buy low and Sell high …..!!!!” Simple… Yet not easy…. Happy Investing