The largest component of closing costs is usually taxes (especially on higher-end properties), which of course are not negotiable at all. Well, the total is not negotiable, but it is possible to negotiate who pays them. You can ask sellers to pay some or all of your transfer taxes, which is another form of "seller assist."
Then you have title insurance, the rates of which are regulated and not negotiable (though you should be sure you get the correct rate; e.g., replacement, re-issue, depending on how long since the last sale).
Then there are title company fees, which are usually low and related to non-negotiable things like government recording fees. You can sometimes save there by closing in their offices instead of the broker's, and some also discount for cash deals.
Sometimes lenders have "doc prep" or other "soft" fees that are theoretically negotiable. Though these days few will waive these, particularly the larger lenders. (These days, you should be thrilled to pass appraisal and close at all :-) Fees are more variable than interest rates; the best way to save may be to shop around. They will give you a GFE (Good Faith Estimate) with guaranteed accurate costs.
You could skip the survey ($250 or so), or defer it until after closing, particularly if there is an existing survey, though you're increasing your risk by doing this. (It's possible there's an encroachment you won't find until it's too late.)
So in general there aren't many ways to reduce the total closing costs, if you and your agent have reviewed the HUD and corrected any errors. But some services are optional, and you can sometimes "redistribute" costs -- even taxes -- to lower the cash you need to bring to the closing table.