On Thursday I published a post spelling out how the rules-as-written work for consciousness and for knockdown and stunning: GURPS 101: Stunning and Knockdown and consciousness rolls
Here are some house rules I use.
Stunning and Knockdown
These rules affect the Stunning and Knockdown rules.
A shield becomes unready if you are stunned; you still receive its Defense Bonus but cannot block. You must take a Ready action to re-grip the shield; this takes one second, regardless of size. Bucklers are treated as weapons, per the rules as written, and are dropped!
(Note: these are derived from the rules for critical failure on rolls to block.)
Stunning, Knockdown are two rolls
Roll separately for stunning and for knockdown. The same penalties apply to both, but you can be knocked down without being stunned and stunned without being knocked down.
Variation: Still use one roll, but if you make the fail the Stunning and Knockdown roll by 1-2 points, you are stunned but not knocked down. If you fail by 3-4, you are stunned and knocked down. If you fail by 5+, you are knocked out!
(Note: I don't do this anymore, it takes too many rolls and too much tracking for big fights, and I found it way too generous to high-HT PCs and NPCs. Even players who track only one PC routinely forgot to roll both rolls, or lost track of "stunned, knocked down and stunned, just knocked down" for their status.)
The penalties for negative HP on consciousness rolls (p. B419) apply to knockdown and stunning. It's harder to avoid being rocked by strong blows when you're barely holding on to consciousness as it is!
This rule applies to consciousness.
Critical Successes and Failures
If you critically succeed on a roll to stay conscious, you do not need to roll again until you take additional injury or take any action that expends FP. This includes Extra Effort, spellcasting (unless it is fully paid for through an energy reserve, ambient mana, or external sources), and FP-draining attacks.
If you critically fail on a consciousness roll, you fall unconscious and your rolls to wake up from any means are at -5. In addition, double the length of time for recovery.
These work very well in actual play, especially the critical success/critical failure ones. Nothing like throwing the Awaken spell on someone who rolled an 18 to stay conscious.