Fickthing a lithp

My best advice for fixing a lisp is to set some time aside and just play with the different sounds you can make with your tongue, teeth, and roof of your mouth. I've noticed that there are actually a variety of ways that someone can have a lisp, all dependent upon the specific place in their mouth that they rest the tip of their tongue.
You will most likely sound like a weirdo, and even feel like a weirdo, so you might want to pick an isolated spot to play with this. While you're driving alone is a good opportunity.
Once you find that sweet spot which generates a good "s" sound, stick with it, and practice that, and start trying to include that in words which use lots of "S"s.
If I have the time, I could possibly draw up a rough illustration of what tongue configuration I use for my Ss, but I fear that wouldn't be of as much help as you just experimenting on your own.

Jam it back in, in the dark.