If you punt for fun then there are only 2 rules:
1. Have Fun!
2. Set a weekly budget of the amount you can afford to lose and NEVER, EVER exceed that budget.

If you are trying to earn some money or even make a living from betting then things bit more complicated. Most of my rules fall under the heading "BET WITH INTELLIGENCE". This does not mean you have to have scored 120+ on the IQ quiz it just means bet with your head and not with your heart. Some things that fall into this category (some of these have already been detailed here but it doesn't hurt to repeat good advice):

1. DEVELOP A SYSTEM AND BET WITHIN IT. It is very hard to make a profit if you change your betting rules each week. This does not mean you cant have a system with a lot of judgement calls built in but you shouldn't place a bet just because it's the last race of the day and your $100 down.
2. RECORD EVERY SINGLE BET. Write down (or put in a spreadsheet/database) every single bet you place and the result. Review your bets, strike rate and POT (or LOT) figures regularly.
3. BE PATIENT. There will be more races tomorrow or next week so if you haven't found the right bets today don't worry about it. Betting is a long-term proposition and you will win some days and lose others - the secret is not to panic if you are having a bad day but stick to your system (or take a break - see next point!)
4. DON'T BE AFRAID TO TAKE A BREAK. If you are losing money (beyond your expected run of outs) then STOP BETTING and review your system/bets. You may need to revise your system and trial again - things change so your system may need to adapt or be fine-tuned. Beware of seasonal changes - things that work well in Spring may not work so well in Winter!
5. TRIAL ON PAPER FIRST. Always try out new ideas on paper first before you put your hard earned (or hard won) $$$ on the line.

I could probably go on all day but those are the most important points that come to mind.

As others have said it is fine to have some emotional bets as well but keep them seperate from your serious bets. I have one betting account that is purely for "heart" type bets - so things like Big Orange's tips, Melbourne Cup or feature race bets, etc.

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"Computers can do that????" - Homer Simpson