I fill the crankcase with oil (via the breather hole) then stand the engine (or model) at a 45-degree nose-down angle, with the cylinder inverted.

This allows the oil to trickle past the rear bearing and pool in the cambox.

Put a rag under the front of the engine because much of the oil will seep out the front bearing.

I leave my engines/models like this overnight to be absolutely sure that the cambox is bathed in oil.

As soon as you start the engine up, any excess will be blown out the breather but this "dose" of lubrication seems to last the whole season long if you use the engine regularly. I have a feeling it also helps prevent rust.