There is no FTP access to WordPress.com blogs and we cannot install anything into them.
1. Use an Offline Blog Editor
The time to backup your posts is while you are writing them. The best method of doing so is to use an offline blog editor to compose your posts in and to publish the posts directly into your blog. if you use one then you have abackup of every post and page you create on your own computer.
2. Subscribe to Your Blog’s RSS Feeds
By subscribing to the rss feeds to your blog and setting them on “full” feed you will have a back-up of every post you publish and every comment posted. If you also wish to backup the images in posts there are different rss feed reader services to consider as only some include them.
3. Periodically Export Backup Copies and Save them to Disc
There is an import and export function in your wordpress dashboard and you can export the posts, comments, categories and pages out of one wordpress.com blog and import them into another wordpress.com blog or wordpress.org install with ease.
4. Backup your blog using Feedburner and Gmail
"Subscribe to my Feedburner email subscription with a Gmail address that includes a label that will be automatically filtered to my archives when it comes in. Using your Feedburner email subscription, any new posts you publish will automatically be sent to your subscribers. Add yourself in with your normal Gmail address plus a label, i.e. [email redacted] and have it filtered to your archives! Viola! Now your entire posts will be emailed to you and saved in your Google account whenever you publish.”