I've seen a lot of posts, but few rules. I'll take a shot at it...
1. Every technique should off balance your opponent to prevent him from completing his attack or starting another one.
2. Every technique should attack the weak points available from delivering your blocks or strikes.
3. All blocks can function as strikes, all strikes can function as blocks. All circular double hand movements should become locks.
4. Attacks to the arms and legs should take place at the joints, where there is no muscle strength.
5. Bending the joints in the direction they normally go can be effected by a strike to the middle joint.
6. Pulling the elbow of your attacker to his own body centerline locks his hip and takes away his balance. Blocking the arms at the elbows upward does also.
7. "Take out the slack" in every technique before striking the target.
8. Step up to your fist, rather than drawing back your arm to punch after blocking. (Moving into your target)
9. When you are held in a lock, move the next joint to effect your escape.
10. Keep your arms and legs in "balance"... knees and ankles slightly bent in stances; arms in "arm sword".
11. Be centered before attempting any technique.
I could go on, but those are a few of the "rules" I follow in my jujutsu system (clearly, not fully described here), but good fodder for discussion.