Every blog on Blogger has a unique ID. The first thing you should do is to find out yours. The ID is a short sequence of numbers, and different for every blog on Blogger. Then, goto this URL: https://www.blogger.com/atom/xx, replacing xx with your ID. You might be prompted for your Blogger username and password. Type them in and you will get the feed of all the posts in the blog whose ID you entered. This is an XML file and you can save it to your harddisk. Keep doing this on a regular basis, and you can be assured that next time you need it, you know where to find it. For easy archiving and retrieval, name the file with the date of backup.
Almost all blog services worth their name offer an RSS/Atom/XML feed to your posts. Although the above tip is specifically for Blogger, similar methods should also exist for others to backup their posts. Please leave me a comment if you know how to do this for other services.
If you did exactly as mentioned above and still did not succeed in saving the XML file, please checkout the following points.
1. Ensure that your blog has Atom feed publishing turned on.
2. Every blog has a different ID. If you have multiple blogs in a single Blogger account, you need to repeat the process for all your blogs separately, each with their own ID. To get a list of your blogs and their IDs, do the following:
Goto https://www.blogger.com/atom. Enter the username and password when prompted and you will get a list of all your blogs and their corresponding IDs. Use the ID of the blogs you need to backup and save them as separate files.