It's hard to give tips on the sitting position because if you watch 10 decent shots you'll see 10 different positions.
The BFTA Technical Skills manual is worth a read, and it's worth looking through some of the excellent GP photo's we had from last season. I would say, build your position from the foundations up - both feet flat on the floor, especially your front foot, get a nice stable triangle and build it from there.
Quite often people that move from HFT to FT with a springer find it difficult to hit stuff. My theory on scopes and springer setup came from a comment Dave Semmens made to me. Now Dave is an awesome shot at both FT and HFT (shh don't tell him I said that, it'll go to his head) and he's no slouch with a boinger either, he's won the NEFTA Hunter series recoiling class in the past putting in performances that many PCP shooters could only dream of. Yet he said to me that he's tried shooting a springer in FT and has always found whenever he tried it he couldn't hit a barn door. So I asked myself what was different for him? - the guy can shoot in the FT position and he can shoot a springer in HFT, what's the difference when the two things come together?
My theory was balance - CoG is important to a springer shooter, but quite often we only ever consider one plane of axis. The difference for Dave was (I believe) that he was slapping an FT scope on his HFT springer and upsetting the balance in the vertical plane. So now what I try to do is keep the weight above the barrel to a minimum and counter balance the weight up top by more weight below.
Just my theory, and bare in mind I've only been shooting a springer for this past year, but that's why I've gone for a relatively light weight Bushnell 8-32 and a heavy stock with adjustable hamster. I don't actually touch the hamster when I'm shooting, but it's adjusted to bring the CoG down to counter the weight of the scope. Could be a bollocks theory, I don't know.