On bolting the seat down in one spot;
I've found, over the years and with a LOT of multi-day trips, that it is sometimes helpful to be able to run the seat fore or aft a notch or two from time to time to re-adjust your legs and back and alleviate the aches and pains that inevitably show up.
I've also done the bolster under the knees thing and found that helpful, too. Haven't done the lumbar support towel behind the back, but I adjust the lumbar control in my DD from time to time and THAT makes a big difference, too. I think that inflatable pad that Danny found is really worth looking into, especially if you're planning on a day-long trip somewhere and especially if you have Speedster seats. My wife's Outback has an inflatable lumbar support with it's own little air pump!
Remember, the Speedster was a dream of Max Hoffman who wanted a stripped-down 356 for people to buy to drive to work during the week and race on weekends. They dropped a lot of weight out of the car for racing and the seats were lightened, too, but you're only in the car for an hour at most when you're doing club races, so the "Speedster Seat" made sense for them. They're far from the greatest on long trips.