Slow riding has a lower limit: your motorcycle won’t go any slower than the engine’s idle R.P.M.
To make your motorcycle go slower than that, one needs use the clutch friction point technique (a technique which is used a lot in heavy traffic, as talked about in this article). That is – let out the clutch lever only partially so as to send partial engine power to the rear wheel, and maintain it at that position. (Recall that the same thing is briefly done while initial pick-up from a standstill, before letting the clutch out fully.)
Here’s a video demonstrating slow riding on a heavy motorcycle:
NOTE: Continuous partial usage of the clutch leads to faster wear and tear, and is not recommended in daily riding except when required (for initial pick up and/or U turns). Engines need air flow for optimum performance – slow riding doesn’t allow adequate air to flow around the engine. Therefore, practising extreme slow riding skills for an extended duration is not advisable. Keep these factors in mind.