You may be a published author with lots of articles and fiction to your credit but don't turn your nose up at sending off readers' letters to the magazines.
If you're stuck on constructing a novel chapter or a short fiction piece, leave it aside and write a letter instead. They're easy to put together - try to find a good quality photograph to accompany it if you can - and are a better way of filling those brain-foggy moments than playing Spider Solitaire (which is what I tend to do when I'm stuck on something!).
As with fiction, get a feel for the content of the mag before sending anything, though there seems to be little restriction on what they take. Keep your letter short, up-beat and interesting. I've recently sent a couple off - yes, you guessed, it's about my recent holiday!
The fee is generally around £25 for each letter printed with £50 for the 'Star Letter'. Some mags will award a bonus prize, too. The turn around time tends to be much quicker than waiting for accepted fiction to appear. So, go on, have a go.