How do I get my card to use turbo mode?

See here for a short explanation of what turbo mode is.

You can set your card to static turbo mode in master, managed and ad-hoc mode. Unfortunately, the driver does not support dynamic turbo mode yet (FIXME: is this still true for madwifi-ng?). This means that non-turbo cards will not be able to associate with your AP, and also MadWifi cards not in turbo modes won’t be able to see it.

Issue the following commands:

iwpriv ath0 mode 3  # this forces it to G-only mode, but you can also use A mode (1))
iwpriv ath0 turbo 1 # enable turbo