Mine very rarely clogs, which I think may be largely because my house has central air conditioning and humidification. We keep the humidity around 40-45% year round.
I did have a clog a few years ago after a six-week trip away. To the best of my recollection, I tried several cleaning cycles without success. Somewhere I heard about the "Windex on a sheet of paper" trick that Sharon mentions.
You might want to look at some of the suggestions at this link: http://(A forum thread)
Or, if that doesn't work, you could try spending $70 for a set of head cleaning cartridges from http://inkmizer.com.
I can't personally attest to either of these, but I did manage to unclog mine with
1. Some Windex on a paper towel sitting under the print heads for a while, followed by
2. About five head cleaning cycles, then leaving it overnight, and then
3. A couple more head cleaning cycles.
Mine has not clogged ever since, even with a couple of waits of four to six weeks between prints.