I managed to up the rate I got paid for a piece recently by showing the editor the Freelance's Rates for the Job page. This is something you can try at home, kids, too.
The editor of a monthly supplement owned by a large media group who should know better (I'll spare them the embarrassment of naming it) emailed me to offer £100 for a 900-word story.
I emailed back to tell him that London Freelance's Rates for the Job site had three crosses (v.v. bad) next to the hundred quid his publication offered, while the lowliest, most obscure supplement in the same group's publications started at £220.
I sent him the address (www.londonfreelance.org/rates) so he could have a peek.
Imagine my surprise when another email arrived, identical in all respects save that £100 was replaced with £220.