One of the annoyances with D&D combat over the years is the seemingly limited nature of common D&D combat resolution: Roll to hit, if hit roll is adequate roll damage. It makes combat feel very...repetitive. Here's some simple guidelines for making combat a bit more dynamic without a ton of extra rules.
Knock Back- If you roll exactly the score required to hit the foe may be knocked back. If the targets level is less then the attackers level/HD they are knocked back(and away) 10'. Foes of same level or higher are a allowed a saving throw to avoid this. The foe is still injured as normal by this random knockback.
If a combatant is knocked back and they have not attacked yet this round they lose their attack. The attacker that scores a knockback on a foe may follow them and draw no instant counter attacks from allies of the foe.
Knock Down- if you roll a 20 on the D20 and a score of 19 or lower could score a hit you may have knocked down the foe. Knockdown is automatic against foes of lower level, those of same or higher level get a save. If knocked down a combatant is now prone, if that combatant has no adjacent allies the attacked and adjacent allies of the attacker may be allowed a free strike against the prone foe. The knocked down foe is still injured as normal. The attacked may choose to stand above the foe immediately if they so choose and keep the foe from getting up if the foe is roughly the same size or smaller.
Don't worry about knocking back and knockdown down on the same strike, knockdowns take precedence.
Trying for an intonational knock back or knock down-
A combatant with superior initiative can try to purposefully knockback a foe. On an intentional Knockback the attack is made at -2, any successfully hit roll will cause but one point of damage and foes of any level are allowed a save -2 for foes of lower level, +2 for foes of higher level (they can see it coming). other wise rule as knock back above.
A combatant with superior initiative can try to purposefully knockdown a foe. On an intentional KnockDown the attack is made at -4, any successful hit will inflict half normal damage and a foe of any level is allowed a save, foes of higher level will gain a +4 to the save (they can see it coming). otherwise rule as knock down above.
Note: Fighters are allowed as many intentional knock back or knockdown attacks as they have levels vs a body of foes 1HD or less.
Additional notes for a bit more brutality and verisimilitude
* if combatants roll 1 one on that save vs knock back they drop the weapon in hand.
* if combatants roll a 1 on that save vs knockdown they are stunned until the end of the following round.
* large creatures are +4 to save vs knock back and knockdown