[fungicord]

The key to a good 's' sound is that the teeth are completely together. As a native English speaker, tonight I tried to make an 's' sound with just the thin part of a pen cap between my teeth, and I couldn't make it sound right. As for the tongue position, it can't be right at the upper front teeth, or you block the air flow and you'll end up with a soft 'th' sound as in "thick." To pronounce "sick," you'll need to pull your tongue back away from your teeth about a 7-8mm (1/4 inch). A good way to practice is to start with a soft "th" sound, and while you continue to make that sound with your teeth closed, begin to pull your tongue back, but don't move the tip it down. Keep the tip near the top and the air flowing in a tight gap between the top of your mouth and your tongue. As you move the tightest part of the gap from the front teeth backward, you'll hear the sound get louder and much higher in pitch. When you go back too far while still forcing the air flow over the top of your tongue, the sound will quiet down again and it will sound a little like a Darth Vader impression. When the sound is the loudest and the highest pitch, that's the ideal position for the 's' sound.

There's certainly more to it than that, but the tongue is a complex muscle and I can only describe so much with words. This is really something that is best learned by mimicking others in-person.