Start prompt treatment. In the official text it is appropriate to use the phrase "Dear Mr./Madam (surname)". You can use the abbreviation "Mr. / Ms.". If you apply by my name, will omit the word "sir" and just write "Dear Sergey Dmitrievich".

In the invitation to an informal event, addressed to a friend, refer just by name or "Dear Elena".
The first sentence of the main text will be dedicated to the actual invitation: "we Invite you to a dinner party", "looking forward to seeing you at the charity concert", "let me invite you to a festive event". Further succinctly indicate the purpose, nature and subject of an upcoming event, such as: "Banquet/reception/conference/meeting on the occasion of the 10th anniversary of our company."

Informal invitation to join a unique and different way: "we Invite you to join us for a solemn day of our marriage", "I want to share its joy" and so on. Importantly, the message was clearly understandable to the recipient: he was called to the event.
Specify the place and time of the event. Sometimes it is useful to attach a map of directions to the destination. If necessary, specify in the invitation the dress code for the invitees.

For themed parties with friends, the invitation should reflect the elements of the entourage of the upcoming holiday, setting up guests on the right wave. Warn invitees about what modern party only in dresses-Solncekljosh and stylish ties, and a disco 90-x - in tights and Varenka.
Don't forget to sign my invitation, using the right to case a formula of courtesy: "Sincerely", "best regards", "With a reliable to see you at the event." In the business text except for the name and surname it is also appropriate to specify the position.

In the invitation to the informal (friendship or family celebration) I like it you used to ask the addressee of the message. If you are registering on behalf of your couple, it is polite to sign the card as follows: "Nicholas and Marina" (for the relatives of the man), "Marina and Nikolay" (loved it the second half).