As per @Bilkokuya 's suggestion I'm writing a new answer which addresses my concern with the currently top voted answer by Jane S and aims to provide a different perspective.
[TL:DR] Transparency is key (as per the usual in any social scenario). The reason you feel bad is because
- you lack transparency on what caused her behavior (with potential of you being the cause)
- you both lack transparency on what the respective other thinks is appropriate
The only way to address this issue is by providing a forum for each party to voice their concerns to get transparency and then negotiate a good solution.
How this abstract sentence can be turned into reality is very situation and personality dependent, maybe my longer answer below helps.
[Should] I message her before the event and say: [Any of the two messages]
No. Apologizing because you feel bad and want to feel more at ease vs. genuinely accepting a mistake which you know you have done are different things. You can't preemptively apologize for something you don't know you have done wrong. [That doesn't count ;D]
Also, what response do you expect to get? Generally you put her into an awkward situation. Either she was offended and she now has to tell you (Its always hard to tell people they did wrong). Or she wasn't offended and you create an strange situation where there is none.
It's a little bit tempting to use moderator tools to just silently remove her from my signup list, so that I won't have to deal with the awkwardness at the table.
Also no. You are dodging the problem by pretending it doesn't exist. On top you are being rude and somebody might argue that it's discriminating.
Staring at women's cleavage is a general problem. What annoys me is that a guy's perspective in this is often overlooked and he is simply guilty. Period.
I understand the argument I often hear from a women's perspective. It goes something like this:
It makes me feel good to wear these clothes.
I feel bad because that guy is staring at my chest (unwanted attention). I can't control his actions. I can't say anything about it, because I don't want to cause a big scene and make him feel bad (even more unwanted attention). That would make me feel even more bad. I don't see another way of communicating to him that he should stop.
and results in her feeling uncomfortable and choosing between two bad options: a) enduring the guy's staring or b) risking to cause "a big scene".
For a guy the argument goes something like this:
I feel a constant, (literally) natural urge to look at a women's chest. Not giving in to this urge takes mental afford which is exhausting. Feeling exhausted feels bad. I can't just stare because it may make her feel bad. Making her feel bad would make me feel even more bad.
This, again, results in feeling uncomfortable and having to choose between two bad options: a) fighting a constant inner conflict against biology, which you can only loose b) risking to offend the women. [This doesn't make it okay or not okay to stare. It just shows the inevitable conflict we guys deal with.]
Who's to blame? The women for using her right to wear whatever she feels comfortable in? The guy for not having perfect solitude against biological instinct? Janne S 's answer seems to suggest the latter since she advises "It's your problem to deal with", which I completely disagree with. In fact there is no single person to blame (even though we'd love that. I see why guys take a punch here).
The problem is that you can't act appropriately due to lack of transparency in: a) realizing that the other person is in distress and b) conveying our opinion on the matter to the other person in such a way that nobody gets hurt.
A good way of acting, in my opinion, is to create a forum where a) we can signal such conflict and b) we can exchange opinions so that the respective other can take appropriate action.
[Edit: I rewrote the section below, hopefully avoiding multiple interpretations this time.]
To that end what I found works nicely is to opt for a fun and lighthearted atmosphere, because what is "inappropriate" shifts. It's the same idea as banter, where you playfully "insult" somebody but not really mean to. Your're probably doing that with your friends a lot and know what I mean, despite me potentially failing to describe it here concisely.
From a guy's perspective the idea is to paint a picture of "let's play a game: I will pretend to stare at your chest and be a douche about it, what will you do. Afterwards we will compare to my expectations and see where we stand". [I can see now how this can sound odd if you read it deadpan serious. Think about it in a satirical way.] The idea is that you create a win-win situation. She can express her actual feelings about this in a safe environment and you can express your concerns. If you run into the actual staring situation later on in the event, you both know where you stand and what is happening. The situation will be more transparent.
Of course, if she is already annoyed or you do this immediately after being "caught" staring, chances are she doesn't want to play this particular game with you. In this case, I think it's best to fall back to blunt, deadpan honesty and ask if you offended her. Potentially with a bit of humor mixed in (that never hurts). I will refrain from examples, because they have only backfired so far [seems to be a lot harder to communicate via text then expected].
However, if you don't have to respond to it on the spot chances are shes willing to play this or a similar game with you. From my experience, I haven't met a person that doesn't enjoy "what if" games. Dang it, she comes to play a RPG which is a big multiplayer "what if" game. Why wouldn't she want to play a smaller "what if" game with you, too?