My best advice:
1. Do not trade all day. Trade the first 90 minutes or so. Many of the best opportunities happen in the morning while price discovery is taking place. If you must trade to the close, come back for the last hour! I personally find that after making a number of good trades in the early part of the am, if I continue to trade its often a case of getting stuck in the chop and giving back the easy money made earlier. Its just not worth it.
2. Limit the number of trades you do. Teresa Lo limits herself to 3 and rarely breaks that rule. She has a ton of discipline. I wish I had inherited her discipline much earlier in my trading career. That one step improved my trading tremendously. If you know you only have three stabs and then you are out, you aren't going to waste them. Trust me on this.
3. Know what you are trading. No one, in real time, can be master of all setups and trading styles. Until you can be consistently profitable day trading, I would pick one or two setups and only trade them. You may go all day without having a single setup appear (unlikely) but if thats the case, don't do it. Even now I have only two primary setups - test
4. Get your mind around why overtrading is happening. If you can quash that "problem" (I believe it to be, been there done that) you will probably succeed in enforcing the discipline required.
Position and swing trading is much easier, in stocks, IMO. You have time to contemplate your moves and are not under the real time gun. You might even question why you want to day trade. Some people I think switch to day trading just to get an adrenaline kick, when in fact they might be far more successful as position traders.
There is something very appealing about getting all that time back. Using only end of day data, orders transmitted at the open, you then get the day to yourself to do something else.
I switched to day trading futures some time ago and do not day trade stocks at all, much to my (stock) brokers disappointment I am sure. I have never looked back - futures allowed me to focus on improving my trading, not trying to find 'the' best stock(s) to trade on a given day.
Just some musings. Oh, a final comment. I believe a person makes it as a trader when the act of trading becomes boring. I tell ya, sitting all day waiting for a setup to happen along is boring!