The most important items that should be on a party invitation are the date, time, place, who the party is for and RSVP instructions. Any special party information should be included as well, such as the type of party, or if there will be a meal served.
In addition to the standard time and place information, touchy situations can be avoided with some special wording. It can be difficult, for example, to have invitees reply with a "yes" or "no," even when the invite requests an RSVP. Specific wording such as "Please let us know if you are coming so we can prepare enough food," or "Please respond by (date) so we know to expect you" may help ensure more people reply.
Another issue that some run into is unexpected or extra guests. Perhaps a child's siblings show up with them, or extra family members join the invitee. Specific wording on the invitation may help avoid this awkwardness. "We are sorry, but due to the theme of the party we cannot accommodate extra guests," or "No siblings please" are two ways to specify no extra people.
Guests naturally want to bring a gift, regardless of whether it is a birthday, anniversary or housewarming party. If no gifts are desired, simply add a blurb onto the invite stating so, such as "Please, no gifts," or "The gift of your presence is enough."