....
The ceiling dose is 400mg, after that any extra codeine is wasted.
No, codeine having a ceiling dose alone is debatable and
not confirmed! If there is a ceiling dose (and personally I actually think
this is!

) it's higher then this, UTS and you'll see it's more commonly thought to be around 700/800mg.
If you'd like to potentiate codeine the best I've found is DXM (Dextromethorphan) - most commonly found in cough syrup, the best being Robitussin for Dry Coughs. This being the best since it has no other active ingredients (although there is artificial sweeteners which
some people could be allergic to and can also cause diarrhoea if consumed in high quantities). I've found 60mg 45mins before dosing then another 60mg when dosing the codeine - however I wouldn't recommend this unless you already have a tolerance to codeine/opiates! DXM has direct interaction with codeine (causing the potentiation).
Another, as others have stated, are first generation anti-histamines. They don't have any pharmacological interactions however can "potentiate" (using this term loosely!

) by their sedating effects.
Here is the Opiate Potentiator thread. However, again, if you don't already have a tolerance then don't go potentiating!
If you find you're not feeling the effects of codeine, you could be in the 10% of the (white) population which cannot metabolise codeine well. Either hypo- or hyper-metabolisers.
i swallowed a whole bottle of codeine freshman year while i was snowboarding unknowingly because it tasted good and it was helping well with my cough.
Umm, in what form was this? Codeine does
not taste good!

It's bitter as all hell!
I also think people here are being a little too paranoid about codeine. Dosing a few times will not make you addicted and destroy your life and making the world end!

Yes, as with
all drugs caution and precautions must be followed. But in terms of opiates, there are
many other opiates which have a much higher addiction potential and much more devastating physically/mentally!
Having said this, I would agree with amapola - it seems like you maybe expecting too much from this drug - it's effects can be quite subtle. If you'll like more information/have more questions feel free to PM me!
.... and play it safe guys!