I find it hard not to be a bit of a backseat DM, but I try hard to keep it constructive rather than destructive. Oddly, even though I get pretty deep into my character's motivations when I am playing, I still find myself thinking about how I would DM things differently.  

I think that this is because I was a DM long before I was every a player and that I have been a DM more often than not over the years.

That being said, I try very hard to limit what backseat DMing I do and to keep it constructive.  For example, when we hit an area where the rules are vague and the DM is struggling, I will often mention how I would handle it.  However, I am always sure to merely present it as an option and not to attempt to throw extra weight behind it.  I also believe that once the DM has ruled, the time for argument has ceased.

I also have a tendency to try to move things along if it looks like party momentum is flagging.  I can't say I always succeed or that I have never been a cause of distraction myself, but I do try to help the DM keep things on track.

Now I should probably mention that the problem you are dealing with is not exactly backseat DMing, but something I call "pushy powergaming."  Heck, just thinking about "pushy powergaming" ticks me off enough that I wrote a post about it on A Hero Twice A Month!

I'm ok with some backseat DMing as long as its rules based stuff. I get a ton of stuff wrong or miss things that are game mechanics because I generally don't pay attention that stuff first. I'm more interested in keeping the game moving and making sure that everyone is having fun.

However, if my lack of carefulness is getting in the way of that fun I expect certain people in the group to gently call me out on it. I am always very excepting of a little nudging when I'm headed down the wrong path.

Where I don't tolerate it, and frankly its never happened to me in our group, is back seat driving with plot points or role-playing stuff. Basically, I just draw the line at when it stops being fun when its happening.

Sometimes its nice to have ideas from the group, but its even nicer when they are solicited rather than just blurted out. Other times it can suck. As with pretty much all things in group dynamics, I like to play it by ear. We have all known each other long enough now that a simple "No worries, I got this on my own" is probably good enough.

Its quite an interesting topic. We have so many to choose from for the next podcast now. You are a writing machine now! :)