Tips I learned doing voice-overs: Try moving your mouth slightly away from the mike. Try not to aspirate on the "S". Experiment with placement of tongue and the position of teeth. It's hard for me to explain but here's an example of how to overcome the popping sound when pronouncing the letter "P". Your lips are together as if you're going to say "P" but a very soft "B" is voiced instead. The listener hears a "P" rather than B because s/he expects to hear a "P". Similarly with "S". How you hold your tongue and teeth will alter the hissiness (?) of the "S". It takes practice.