I usually use one of the 'trees' too, and especially like the black ice one.
One of the ladies I work with uses a sta-soft refill (here in South Africa we get fabric softner refills in a plastic pouch type thing that looks like the pic below). She just bombs one of those under her seat and the smell lasts literally for months.
Of course having your car smell like fabric softner may not always be so appealing to us manly men.
and for those of you with kids, there is a real risk of them picking it up, squeezing it, and you having fabric softner all over the interior....