Same as above. Don't Fight the Tape. Same as above. If you are going against the trend, you have to have a very good reason why. Don't Put All Your Eggs in One Basket.
All professional traders have a self-imposed limit that they can invest in any one trade.
Everyone gets it wrong on occasion, and you don't want your mistakes to deep-six your savings. Never Sell on Monday. When bad news or good news hits over the weekend or after the market closes, sit down and think what it really means. Stocks will always bounce or get sold off temporarily after the news hits. If you have to get in or out NOW, you are probably paying too much or getting too little for your trade.