Do you have some post that you would like to display in your sidebar? Maybe its a welcome note, or something that explains your site. Anyway let's get started.
First thing you should do is to go to your Dashboard > Layout. You should now see the outline of your blog. Click 'Add a Gadget' in the sidebar. Scroll down 'till you find the 'Text' gadget. Well, add it, just hit the + button beside it.
Now you should see an editor similar to those used to write posts. Type in your text there (whatever it is), format it if you like, and hit on the 'Save' button, and position the text where it fits you the most (just click and drag). Now you have a text or two that could not be overwritten by your ordinary post, because it's in your sidebar. If you want to write more just repeat the steps given.