I always thought it was hard to judge proper torque with a folding allen wrench set. I assume yours looks something like this:
What might feel like a lot of torque on one of these may feel pretty loose on a longer wrench. It's hard to tell.
If that fails, your seatpost may be a little small for its size. Besides getting a better-toleranced seatpost, you might try one of the new friction compounds offered by a few manufacturers (like FSA and Ritchey). Ritchey's looks something like this, and it's supposed to reduce the amount of torque necessary to keep a part from slipping:
You'll have to pardon me if that's what you meant by 'assembly grease'.