Say you're trying to tower dive an opponent as a Lumbrr or something. Just go in first (you have to hit enemy to aggro tower), hit the enemy with S2 ult whatever and just stand right inside the tower range.

That's all you gota do, don't get out of tower range when you have full health (this will cause the tower to aggro someone else), don't chase the enemy with your S1 or AA (you'll probably die from taking too many hits), just stand right inside the tower range and move out when you about to die.

Bonus tip: always buy rock shield.

Since the introduction of new support items ages ago, you can switch support items without using gold. This means once you use your support item passive (cleanse for example) and switch to the passive support item without any penalties. Once you do that, the rock shield active ability becomes available instantly.