What are the commands to make a Juniper switch send netflow data (V5 or V9) to a netflow collector for bandwidth monitoring?
I presume by switch you mean the EX2/3/4000 range, in which case, it is not supported.
sflow is supported if that's what you would like, though it's not implemented on the version we use. I heard it was to be implemented on 9.5R.
There is a "hack" you can do by mirroring a port and then running something like pmacct or ntop on it, then generating Netflow records from the mirrored data, but this isn't a sustainable scenario.
The EX8200 series will support sflow and Netflow (with a cflowd export line similar to other Juniper products).