stop doing just a little bit of what you need to do, and do EVERYTHING that you need to do.
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Make sure that your files on the server are clean. That means deleting and reuploading. Files that you dont replace, should be swept.
Check for files that dont belong, directories that dont belong. Image files with changed timestamps -- look at those. Its VERY common for there to be scripts on sites that are named in such a way to mask the fact that theyre scripts.
Be suspicious, when youre looking at things.
Look at your permissions. Do you have world writable files? Any world-writable directories? Are they necessary?
You need to check your database. Look for rogue plugins being loaded, look for rogue users (specifically look for a user named wordpress). You will NOT see rogue plugins or rogue users in your wp-admin/ area. You need to check your database.
Make sure ALL of your plugins are current.
Make sure your wordpress is current.
Change your mysql password that wordpress uses (update your wp-config.php with that new password). Especiallly important in cases where you see changes to your mysql database.
Change any admin level passwords on your blog. Change your ftp password(s)
Scan your local machine for malware.
Look at any other software thats being used on your site. Is it current?
That's just an outline and not a complete list.
There's quite a bit to do, but it's all necessary.
If you cant do it all -- get help.
Then there's this:
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