Most of the time the problem of the default text being overridden is caused by people using the rich text format posting option. It's highly recommended that you don't use the rich text posting option (because it's evil.) Bad coding on a RTF post can result in broken layouts and a lot of people getting cranky.
In place of using the RTF option I would recommended downloading a LJ client, there is a list of them here.
Otherwise, here is a handy posting template containing all the coding you need for your post. Just copy and paste the text in this box into the HTML posting option, enter your own text, insert the link to your fic where indicated and you are good to go.
If you are unsure about anything ask the mods before posting. If you have any comments, questions or concerns, pick a mod and PM them or drop us a line in the suggestion post