According to Theme or Motif
Most hosts actually set a theme for their parties. You should check the invitation and see what theme would be followed for the party that you would be attending. If the party would have a Hawaiian theme, your gown should be related to Hawaii somehow. You can wear a gown with some flowery patterns, you can choose party wear gowns that has Hawaiian related colors, you can wear accessories that are appropriate for the theme, etc.
Some hosts would simply set a motif instead of a theme. They may instruct you to wear a certain color or color pattern for your outfit. If the motif is black and white, you can use a gown that is either black, white, or both.
According to Other Attendees
You should also consider who the other attendees would be when looking for party wear gowns. If the host invited young adults, then you can choose a gown that exudes youth. If the host invited people who are of high professions, then formal gowns would be better.
According to Location
The location of the party would also play a great part in your decision. Is the party to be held at a hotel? Is it going to be held at a beach? Is it going to be held at the residence of the host? Would it be held at a rooftop?
You should dress appropriately in relation to the location or venue of the party. Wearing a long gown at a pool party may just get your gown ruined. Wearing a cocktail gown at a party in a hotel may make you seem underdressed. The venue of the party would be indicated in the invitation so it would be better to check this for you to know what party wear gown to choose.
According to Personality
Aside from factors that are related to the party that you would be attending, you should also consider your own personality. You would not be able to enjoy the party to the fullest if you are not comfortable with what you are wearing. If you are not someone who would show too much skin, then don’t wear backless gowns. If you are not a person to wear bold colors, then choose black gowns. If you are not a person who would wear body hugging gowns, then choose a party wear gown that is not that fitted to your body. You must love your own style and don’t be shy of it.