T(h)anks to all,
I think Cookie Sewell's Su-152 (Mar 04 FSM, “Working with limited-run kits”, pp 54-57, esp. 56 — Su-152's used the same chassis as IS's after all) shows the best way — pin the tread sag down with piano wire rods running crosswise all the way through the hull, slightly below the tops of the return rollers, then hide the rods with a little paint.
I wish I could say “I've done it and it looks great”, but because the photo I'm using as my primary reference has 6 infantry in greatcoats and felt hats that I have no desire to try to sculpt, I'm still waiting on Military Hobbies to deliver a set of Siberian Riflemen (Revell-Germany #02516) before I get serious about moving this kit out of storage and onto my workbench.
Occasional factual, grammatical, or spelling variations are inherent to this thesis and should not be considered as defects, as they enhance the individuality and character of this document.