grammar question~~~~

These 2 sentences are different in meaning :

before the presentation day / before the presentation – sentence 1 clearly stated that it was a presentation day, while sentence 2 only say presentation. In sentence 1 we know very well that the verb (discuss) happened before the presentation day. While for sentence 2, it can mean only one hour before the presentation.

Sentence 2 told us that you will dress up for giving your presentation. Sentence 1 does not have this content at all.

For grammar :

Sentence 1 :
we should use which style, not what style, because which is used as the object of a verb (discuss) to show what thing you are referring to.
The latter part of the sentence should use inversion, and we should use dress up. So sentence 1 should be changed to :

1. Before the day of presentation, we discussed on which style should we dress up on that day.

Sentence 2 :
How we should dress (up) is acceptable, but we cannot say “be dressed” because this is a passive voice, while the fact is that you are dressing up by yourself, not by other people. So sentence 2 should be changed to :

2. Before the day of presentation, we discussed how we should dress to make our presentation.

What about saying this:

"The day before presentation we discussed how to dress up."

It seems we don't have to emphasize "on that day" if they mean the "presentation day" only.

2008-10-25 09:03:01 補充：
if the meaning is not one day before, however, i suggest changing "The day before presentation" to:

Shortly before / Two days before / A few days before / A week before, etc.

I would like to say this way:

We discussed the dressing style for the coming presentation.