How to effectivel push-back and negotiate goals with boss

H/G
What has worked for me before is to have a clear communication to my boss of what it is that I am doing.  I find it useful to show him what I think the priorities are.  Whenever something else is "served on my plate", I make sure we both agree on what it's place on the priority list is.  Next, priority #1 is to schedule tasks on my calendar and focus on completing my tasks in order of priority.
Delegate when appropriate!  When in doubt- delegate.
Cheers,
Raúl
 

It's a good question without a great answer.
It's unlikely you're going to reset your boss's expectations.  It's possible, but it's pretty unlikely in my experience.
If you have people working for you, the first answer is, as Raul suggests, to delegate.  If your first response to this suggestion is that you DO have folks who work for you, but you can't delegate, you need read no further and the issue your unwillingness - nothing else - to delegate.  (There are rare legal exceptions with work rules and unions, but I'm guessing those don't apply.).
The idea in the paragraph above carries further to if you don't have directs - that means you are tat the bottom of the org (don't let the phrase imply that I think you are minimum wage, it's a structural insight, nothing else).
If that's not the case, keep track of the work you do, and report on the work that you ARE doing.  When you get more work, prioritize appropriately. 
And here's the key: decide what you're not going to do, and don't do it.  This is what we recommend in Delegation casts and the Juggling Koan cast (hugely popular, by the way).  Since I know you have work left over each day at work, I know you're already doing this, but you're using time - "I'll get to that later" - rather than deciding what to get done.  You're just making your queue longer...and in my experience, worrying about the queue makes all the other work we do much less efficient and effective.
This is almost math - you can't do everything, and your boss keeps giving you more.  That combined with your statements about work hours ( I agree, you're topped out) means something is not going to get done.  Be smart and choose what that's going to be.
I'm not saying this phrasing works for everyone, but it does for me and might for you: I'm going to get in trouble for stuff I'm willing to get in trouble for.  Put differently, if you're going to get fired (you're not) get fired for something you can defend.  DECIDE. 
Oh, and yeah, there's risk here.  But you have risk where you are, too.
We suggest this is a smarter risk.
Hope this helps - glad to serve.
Mark

H/G,
 
I have a similar situation that had happened to me in the last year.  I was moved under a new manager who did not have experience with my products or markets and, as such, was unaware of many aspects, and volume, of the work required to maintain and grow the business.  This issue is compounded by the fact that I was now on the 'blue pill' side of a Matrix organization with resources now primarily assigned to me by this new boss.  My previous boss, an MT acolyte, practiced the trinity to some extent.  This new manager did not.  As such, once I listened to the 'professional update' cast, I scheduled a weekly 'update' where I would lay out all of the tasks and work I was required to do to maintain the product line but also to develop new products.  Over time, he saw there was much more to my responsibility that initially intended and the demands for additional tasks tapered off to a reasonable level.  Perhaps you could try something similar.
 
All the best,
DW

Get ahead of the curve. Suggest extra work for yourself to your boss. Just be strategic about what you assign yourself.
I had one employee really impress me by highlighting some of the inefficiencies in their work load. He came up with a plan to eliminate them and asked for the time to do it. This had the benefit of showing me he could see the big picture and solve problems on his own. Additionally, it got me on board with the work. As a result, I found ways to give him the time he needed to make the improvements. Once he had cleaned up shop, I was able to delegate more work to him than anyone else in the group. This resulted in significantly more face time in meetings with executives and a role on a key product. When there was an opportunity to promote someone in the organization, senior management came to me and asked for this person specifically. It worked out great and it was due entirely to the individual contributor's own initiative. 
My 2 cents.
-hnut
 

I'd also suggest you think about why he might be doing this and then use that to your advantage.  Maybe he is being overwhelmed with new work by HIS boss, in which case your team can help with the load.  Or, is there a theme to the types of things he asks you to look into?  Maybe there is a certain business problem that is keeping him up at night or maybe he is one of those creative managers who has lots of ideas and needs the team to help think them through.
The group I work in actually exists largely because of this.  My boss's boss used to pass many requests for non-standard things to my boss (we work in a very standardized IT environment..)  Orignally these things weren't in "the plan" and were considered extra.  My boss figured out that this was because, due to business requirements, his boss NEEDED to get some non-standard things done and no one else coul/would take them on.  We were able to take on the extra load and evolved that into a new charter of being the team that will take on anything if management needs it.  But to get there, we had to look beyond the burden of extra workload.