When going to a party we are shooting for sexy, but also for easy access. A sweater-dress with a hot pair of thigh high boots (with or without leggings) can be nice for cooler weather. She can always take the leggings off at the party if it's a chilly day.
For a woman with a little belly, something that comes tight just under the breasts but hangs down from there works well. I'll usually go for either an off-the-shoulder or deep V neckline to highlight the Mrs better assets ;-) I think the basic look/style is called "baby doll". A tube top upper that hangs down from under the breast can be both sexy and provide easy access too.
In warmer weather, a sun dress can work well with either high heels or what I call "fuck me" (platform heel) strappy/cutout sandals. We both like the look of some of the longer asymmetric dresses as well. It's rare that the Mrs will wear pants to a party unless it's really cold out. If she does, she'll still pair it with a shirt as described above or a nice sweater dress.
The length of the shirt/sweater dress is usually long enough to just barely cover her ass if she decides to wear just that at a party, which is usually the case after we've played and she doesn't want to put all of her clothes back on.