This tip comes from Eileen Meitzner. She has a good Rowenta iron that had begun to spit and was worthless to her. She cleaned it with vinegar, and it is like new again.
For stubborn clogs, fill with vinegar and repeat the process as often as necessary to clear out the particulates.
1. Fill the reservoir full of white vinegar.
2. Heat for one hour. Very warm but not highest temperature. You might want to plug this in out in the garage to avoid the vinegar smell in your workroom.
3. Place paper towels in the sink. Use the flush button and flush out the vinegar. You should see little particulates caught on the paper towels. Flush with water several times until it stops spitting particulates.
She also recommends that you use distilled water - not tap water. Tap water contains chemicals and minerals that build up inside the iron. I know that instructions often indicate that you should not use distilled water, but I have to agree with Eileen here. Distilled water does keep the iron and steamer cleaner.