It can often be difficult to get a clamp and bars to fit together if the clamp is a tight fit for the bars. You don't want to use grease to make them fit together as it will be difficult to get a secure fit later on after you tighten everything. The best method for expanding your clamp for easily fitting it on to your bars is to use the penny trick.
The method outlined below will work for standard clamps, HIC clamps, SCS clamps, and it might even work with the Phoenix iHIC bars/clamp system, but I haven't had a chance to try it out with those yet.
As always, use T-handle allen key wrenches rather than L-shaped allen keys. You can buy a full set of T-handle metric allen keys for less than $10 at Harbor Freight and I highly recommend that you do so.