, including a % sign in your search really does nothing. The search php code strips the % sign specifically, but adds it again in the actual SQL query.
$check_keywords = str_replace( "%", "", $check_keywords );
$post_like = " LOWER(p.post) LIKE '%".trim($keywords)."%' ";
I am not really seeing anything in the code to provide for + or - signs.
If you want to search all your own posts (i.e., if you are looking to see if anyone has replied to any of your posts), you can go to the Search section, type your name in under member name field, check match exact name only, and then search with no keywords.
If you want to see all new posts since your last visit, click the "View new posts" link (as if that wasn't obvious).
If you want to search keywords on a page, just add to the address bar "&hl=keyword" at the end of the present url. This will highlight all instances of "keyword" on the page. (Note, until the server is on php5, and IPB is coded for php5, this is case sensitive and thus Credit and credit are not the same thing. They will match in a search, but wouldn't be highlighted by this example).
To the admins: if it hasn't been done already, if you FULLTEXT index the posts in your ibf_posts table, and set IPB to use FULLTEXT in the ACP, this will make searching the forums a LOT quicker.
On a site like this, with a DB size likely to be around 60-80 MB or larger, you would likely need to FULLTEXT index from command line, through SSH/shell access. But if someone COULD do this, would be a great improvement on the searching system, probably the best thing you could do to it behind the curtains.