Shooting at your feet while sliding helps a ton with speed. The items you can get in the secret level can increase slide speed + decrease slide friction, so those help as well. I will say though, I got plat on all four of the main levels without very much movement tech or gear. It tends to be more about knowing the correct route to take, so my biggest advice is to explore all the sealed areas once you get gold and then go for plat once you think you've found a more efficient path.

starter gun is atrocious for building up horizontal speed now in my experience, The krifle (ak-47 type thing) is very good and is my new go-to for straight-ish lines.