I once read that sermons are like meals.  Some we will wished we hadn't had, some we will remember for a long time, maybe even a lifetime, but most we won't really remember yet combined, they keep us healthy.  We can also use analogies like "hitting a home run" with the sermon.  A batter doesn't hit a home run every time yet he gets up there to swing every time.  At the end of the season some of those swings are remmbered for their impact (the grand slam), some might have been better better off not being done (swung out for strike three), some we will wish we had done better (got caught watching a called third strike), but most we will not likely remember yet they made for the whole season.