Has anyone used Supersocket with J2ME clients? I would like to know what you guys implemented to maintain a good connection (despite having no control over the network). Any best practice?

TCP already provides keeping alive approach, but you also can design your own keeping alive strategy in application level. I think your own keeping alive should be driven by client side.

Tried both the default keepalive and a custom implementation where a heartbeat is sent by the AppSession every x seconds, but the connection is still not reliable. I guess it has to do with the network. I wonder if moving the heartbeat to the client will have any difference.

You cannot ensure your network is connected always, but you can reconnect after the connection is dropped.