Thomas DeMark's Sequential (tm) System 1999
 
Basically the criteria are as enumerated below, however, to appreciate the rational and nuances
 you are urged to read Ch. 7 of his book. 
The system is divided into three distinct stages, A. Set up, B. Count Down, and C. Entry. 

A. Set Up Criteria 

1. For a Buy signal:
Nine consecutive daily price closes which are lower than the close four days earlier. 
For a Sell signal: 
Nine consecutive daily price closes which are higher than the closes four days earlier. 
2. An Absolute Prerequisite 
The day prior to the first day of a nine consecutive set up sequence must: 
for a buy signal have a close higher than or equal to the close four days earlier, 
for a sell signal have a close lower than or equal to the close four days earlier. 

B. Count Down 

For a buy signal, following the successful completion of the count down 
we wait for thirteen closes which are lower than the low two days earlier (
not consecutive, in fact extremely unlikely to be so). 
For a sell signal, after successful set up, 
wait for thirteen closes which are higher than the high two days earlier. 

C. Entry 

On successful completion of set up and count down we now have a choice: 
a) Enter on the close of the day that the count down is completed, i.e. 
the close of the thirteenth defined close. 
This is the most aggressive and highest risk entry but most closely approximates 
the exact high or low of the trend about to be reversed. 
However, it is the entry most likely to be whipsawed if the Sequential fails. 
Entry after reversal 
b) On a close higher for a buy (lower for a sell) than a close four days earlier 
after the count down has been concluded, i.e. post reversal. 
c) On a close higher for a buy (lower for a sell) than the high (low) two days earlier 
after conclusion of the count down, i.e. a compromise between a) and b).