These are all good pieces of advice. Sanding/prep for painting is the real key to fine paint job. Rushing through will leave you disappointed.
I tip I learned from my Dad I use regularly and find it as valuable as block/wet sanding.
Use the flat of your hadNd and run it over the area you are working. Y
If you concentrate you will feel valleys and ridges. You may have to sand more or apply a bit more body filler, whichever is appropriate. Then you block sand as a finite step. Another thing you can try is wearing a thin glove...vinyl etc. You will be surprised what thr glove allows you to feel that your bare hand may have "ignored".
