Its hard to put into one sentence what's really important so I'll have to bore you with a few more things which I think are the most important for me whilst daytrading....I've been doing so for 4 years btw.
Don't run any losing trades thereby eliminating emotion from your trading.Always try to trade in the direction of the underlying market ie.on up days, buy the dips rather than short risers for eg.
Try to be flat ahead of important data or at the day's end.
Keep it simple, in my case, 1 screen and charts in my head for the stocks I follow.
Trading successfully imo is about judging human emotion ie. make the first move before the average investor/trader does.Be brave intially.
Naturally, other successful traders will disagree with some of my points but as they work for me I'm unlikely to change!!
If pushed , the control of emotion, is likely the most important parameter but that in itself encompasses most of the other points.