A move below the lower trend line will be a signal tht the current up move is facing a correction or whatever. This is simple technical analysis, but effective.
If we follow this method, then we are unlikely to sell at the very top. Prices will have to come down about 150 points before a down move is signalled. Now, think about this: is this so terrible? In an uptrend, do you really want to goshort every time the Nifty moves down just a little bit?
As I write, U.S. markets are down. It is possible that this weakness can continue till the close, and, may then affect the Indian markets tomorrow. If you are a day / swing trader, you may like to use the 15 minute rule to go short in the Nifty. You should also keep some rules with you on the short term trend. What will signal a change in the trend from up to down? I have given one rule here - the channel. You must have some method of your own. So far the short term trend is up, short positions carry high risk, so keep volumes down, and follow your stops.