I've always had this issue, but I'm trying to bike a lot more this summer than last year as the wife got her own REI ebike. I cannot get the seat to stay at the height I want.
It has always slipped in. Basically, if I have any sort of sideways tension on the set, like, you know, pedaling, then the seat slips sideways and down.
I only weigh 215 lbs so its not like I'm overweight for the bike.
I have both of the seat clamps as tight as I can. I tighten them to the point where I can't actually close the clamp, then let off a half turn, and then close the clamp. The bike seat still slips down once I start riding.
Any advice or ideas?

What are you all wearing on your commutes?
We've hit the rainy season here in the PNW and its discouraging me from getting on the bike. I have a 10-12 mile commute (one way) mainly on roads competing with 18-wheelers and other non bike friendly individuals.
On my old analog bike, I was decked out in rain gear and clip on shoes with toe covers or full shoe covers.
But with my Radwagon, I am unsure what to wear on my feet. I have been wearing some running shoes, but that is not going to stay dry. Also, my normal rain gear is designed to breath as I'm working hard, but obviously I'm working less hard on the new bike.  So, sometimes I'm cold, and sometimes excessively warm. Part of getting the bike was to reduce the amount of changing clothes I had to do at work, but that does not appear to be working out.
So, what are you wearing? Feet wise? Clothing wise?
What have you tried that was a complete disaster?