Breaking off a big fish is always disappointing, particularly if you think it's the knot.  Still, it's awesome that you hooked a big steelie.

I haven't had this particular problem since I did 2 things to the clinch knot (note: I don't even bother with the improved one).  1.  Lubricate the knot with spit so it cinches up really tight, 2. Leave at least 1/8th inch tag after cutting the excess.

The lubrication basically eliminates slippage altogether and the 1/8 inch tag is just insurance.  When I was fishing with Lonnie Boles (The Fly Shop) on the Lower Sac this year I noticed that he doesn't even bother to cut the tag off.  After thinking this over, it makes sense because if the fish isn't bothered by the main leader connection, why would it worry about the tag?  I just cut it to 1/8 inch for esthetic reasons. 

Also, a guy I know that is a wonderful steelhead angler uses the Lefty Kreh loop knot which is awesome for big fish.  This is the knot I used in Baja and didn't have a single break-off.  It is also easy to tie.

http://www.netknots.com/html/non_slip_loop_knot.html

I use loop knots at Pyramid using 2X and 3X fluorocarbon.  The best loop knot I've found is the Rapala knot with 4 turns.  My testing (at the kitchen table) has shown me that the Rapala knot is much stronger than the Improved Clinch.  The main problem with the Rapala knot is that it takes a bit of practice to tie it consistently, especially if you want a small loop.  I've tried a "non-slip loop knot", but it was not as strong as the Rapala knot even though it is tied in a very similar fashion.  As I have mentioned is a previous post, knot testing is very complicated since the "strongest knot" is dependent upon hook size, the type of tippet and the number of turns used.