Almost impossible during high season (Dec to Easter). Other times, go to Barefoot Beach Preserve, off Bonita Beach Rd, $8 per car or Collier County beach permit. Park in south (gravel) lot. Hike aprox 15-min to where families thin out. Thongs permitted, discrete topless/nude possible. Rangers patrol on Gator vehicles which can be heard in time to cover-up.
Tiger Tail beach on Marco Island. Just walk out onto "the tail", it's a thin sand sprit; just walk until you're comfortable. <img src="http://www.traveltalkonline.com/forums/images/graemlins/Groovin.gif" alt="" />