The highly varnished seat can make it a bit easier to slide but this is an issue on a most small boats, even on fiberglassed seats. The only way to prevent this from happening is to be prepared by bracing oneself when in breezy conditions.  The wind is unpredictable as you found out and it just takes one gust to increase the lean angle substantially.  Knowing this is a possiblity you need to be prepared to lock your leg against the opposte gunnel and act like ballast and lean your body into the gust.  

Welcome to the world of single-handed dinghy sailing!  Part of the "fun" is trying to anticipate and react to the wind.

Generally speaking, you should have the jib cleated off unless you are actively tacking.  Change the boat's heading in responce to wind shifts to keep the jib properly trimmed.  Adjust and recleat the jib as needed to get the course that you want.

Play the mainsheet to depower the main and keep the boat on its feet.

Varnished seats are a problem but a pair of specialty sailing shorts may help.  I like Deckbeaters by Zhik because the outer layer is rough which results in more friction.  They also have foam padding to protect your nether parts.

You did not mention how your deck is finished.  Varnish looks good but is bad for friction.  I suggest some kind of non-skid and a good pair of deck shoes.  Your best connection to a dinghy is through your feet.  Maybe add hiking straps if needed.   

Oceanluvr and Mark N., thanks for your responses.  For aesthetics, I applied seven coats of varnish to my thwarts.  They look beautiful, but they are as slick as a block of ice.  I confess that I'd not sailed a dinghy for a couple of decades -- a J-22 has been my small boat of late -- and had not been in one as light as the NE Dory since my Sunfish days, nearly a half century ago.  The good news was that she sailed flawlessly, the skipper's rust notwithstanding, but it's more than a little disconcerting to enter a slide to leeward that is reversed only by grabbing the windward gunwale with both hands and pulling one's self hard to windward.  I'm used to the seat of my pants remaining more or less in one place with only the force of well braced legs to help weight shift.  I shall look into the Deckbeaters as well as some type of rubber mat that might hold things a little better.  Again, thanks for your input.  It is appreciated.         

   Consider the Seadek precut kits that CLC has for all their smallcraft.  I put a kit in the bottom of my skerry and it's miles better.  Also, both the skerry and NE don't have side tank seats, so the most secure place to sit is in the bottom, not on the seats.  I use a Type IV flotation cushion under my butt.  It keeps me further off the water in the bottom or as a back rest.

Mummichog -- Having just completed my dory, I'd like to go simple until I have all of the kinks worked out and I figure out how I want things to work.  In the meantime, your idea of using a flotation cushion is first rate.  I used to use one in a 10' sailing dinghy I had and that worked great.  I'd forgotten about that.  Again, thanks.   

   I bought a couple of 1/2-inch thick closed cell foam gardener's kneeling pads from home depot to use as cushions in my dory.  They have proved indispensable, especially on long rows where they are just the right amount of cushioning for your butt (a flotation cushion is too fat).  They also stick pretty well to the dry thwarts to prevent you sliding while sailing.  However, if the thwart is very wet, they will slide.  

A hiking strap is also key, i have a 1" webbing strap strung tight between a soft padeye on the daggerboard trunk and a padeye epoxied onto the deck just in front of the aft-mid thwart.  Works well and is removable for when youndont need it.