Some tips probably stated above:
Use and abuse the gun, Russian heavies from the KV-1S to the T-10 (old IS-8) rely upon their gun and mobility to out trade opponents, and preferably outplay by flanking, baiting a shot, etc.
The armor is good, for a medium. These heavies are pseudo heavies also named heaviums due to having the best of both worlds. They retain some mobility from mediums while gaining a punchier gun and better armor. However this armor should not be relied on too heavily unless you have placed yourself in an advantageous position as it can pull some bounces off but isn't expected to every time.
Mobility wise they are fairly decent trailing behind mediums and lights but staying at the head of a heavy push or even pulling ahead. This is a double edged sword as you meet other fast tanks and out trade them easily but beware of getting out of position due to that speed. Also you need to be flexible whenever possible so try not to lock yourself down in a position you can't fall back from easily.
I have 1932 battles in tanks from the that line (KV-85 to t-10) so I have a bit of experience in these tanks.
Also i'd recommend free xping the top gun for the IS-3 and T-10, remember the guns make these tanks and shooting gold all day is a quick way to go broke with Russian (.46) accuracy.