Use an informal and tongue-in-cheek style to keep your readers interested when writing a letter or blog about your travels. Start your letter with a warm general salutation and a brief explanation of where you're going and what you expect to do while you are there.
From there take your readers on a day by day recap of your adventures focusing on the high points in a conversational style tone. If you're using pictures for your blog be sure and explain the content and feel free to add humor to keep it interesting and engaging. Be sure to point out and describe points of interest that you know your friends and family will be interested in.
When the trip is coming to end, bring your letter or blog to an end in a manner that will wrap everything together. If you have pictures on your blog, try to get a shot that will be fitting such as a view of a tropical sunset if you're having a beach vacation or a shot of the airplane that you will be flying on to bring you home.
With the right pictures and words, you can whisk your friends and family off to far-away places that you visit and they can live vicariously through your globe-trotting treks and travels and linger on your every thought and word either through your letters or your online blog and picture campaign.