Holding Long Term Rental Properties

[Jack Gauntlett]

Hey BP members, I just got done reading the book "Timing The Real Estate Market" by Robert M. Campbell and have a question about holding long term investments. In this book, Campbell emphasizes the fact that in order to succeed in real estate then one must buy low and sell high (which is obviously the goal of almost all investments). My concern is if it is worth selling at market upturns or to try and hold through a downturn. I ask this because I want to one day live off of rents that I receive, but I am worried that if I sell all of my properties after each cycle then I would essentially have to start over at the beginning of the next cycle in order to get my passive income back. So after all that rambling... my main question is, is it worth it to hold on to my properties through all cycles and keep collecting cash flow or should I sell at each uptrend then rebuild my portfolio when prices are low? 

I appreciate any and all replies, Thank you!!, Jack