As well as the technical means mentioned in other answers, you should also consider "working the mic" - understand how various syllables produce air out of your mouth. Position yourself slightly off-axis from the mic so the blasts of high frequency sound and high speed exhalations don't hit the mic's diaphragm head-on. 30 or 45 degrees off-axis can produce suitably good results whilst helping to mitigate sibilance or excessive plosives without totally ruining your high frequency pickup.
Sibilant sounds produce a large amount of powerful high frequency energy which sounds OK on its own, but as soon as you begin to mix it in or crank the gains it quickly becomes earsplitting! You say you have a lisp so the problem is more obvious (with the mixed, distorted "s" sounds); there's a few things you can try which don't require much of an investment.
Fit a windsock or pop shield to the mic; it's a good subconscious reminder to not "eat the mic" - and anyway, you shouldn't be too close to it (about a stretched-fingers-apart handspan away is good for most decent mics). You can make your own pop shield for a few quid / dollars if you're in a DIY mood.
What mic are you using? Understanding how it picks up audio (and its pickup pattern) is very useful. Record yourself saying either the same thing or a sample sentence over and over whilst moving slowly around the mic (or moving it around you). Listen back afterwards so you don't just hear yourself whilst you talk, and analyse what you think is both the most optimal sound for you and what sounds good to others.
All de-essing is fundamentally a sidechained compressor/limiter over a specific band of frequencies (4-6 kHz) with a high-Q (sharp-curved) EQ cut applied to those frequencies when they exceed a specified level of gain. The de-esser's 'wet' signal is then blended with the original 'dry' signal until a happy medium's found. You can do a quick 'n dirty de-ess by getting a shelf EQ loaded, cranking the Q to about 10 or 15, turning the EQ up to about +20 dB and sweeping slowly between 4 and 7 kHz whilst you repeat a very sibilant word ("sibilance" is good!) When you find the problem frequency, it'll become very obvious - at that point, take the EQ's gain back to -3 or even -6 dB (to taste).
Once you've done radio (or audio presenting, in my case, in a studio environment) I found that I actually started de-essing myself by deemphasising how severely I anunciated my "ess"es (slurring them instead of crisply pronouncing them).