Notice how the Dow has stopped its ascent right in the gap created on the open of 15th March? And now its fighting itself right inside that same gap area - very excellent in a weird technical way.
Annoyingly this website won't allow anything less than 15 minute charts to be uploaded, so hope any newer day traders are using the 1 minute chart and slinging a trend line under the first two/three lows as the rally kicks off...almost always you'll get three good connections on that line as the move progresses and you'll know it's trustworthy. Then, once the impulse wave has lost it's power (usually after the 4th hit on the line (ie the 5th) and the index wriggles sideways and below the line, it's an early signal to exit and consider going short for a while if it's looking overbought.
Either that, or keep raising the stop to just 3 points below the last little low, but this is a little slower to exit and almost always from a lower level than using a simple trend line on the one minute chart.
Bleeding obvious, really.