you should decide 'how much' of a profit / loss you're will to accept... then be ready to ride a roller coaster.
I've found that when I get greedy with profits on daytrading is when I get hurt. Of course you have to factor in buying / selling commissions too when you look at profits as well.
I think the best thing I found was the same advice given to new players - think short term. If your stock has gone up 4 points, don't get greedy, sell and move on.
On the downside.. if you find you're losing too much on a stock that's going down, maybe it's best to cut the losses and move on.
Daytrading can be a real roller coaster ride if you're not used to it, so I would say you may want to start slow - see how your instincts work, and then decide if you want to go for more...
Good luck.