I clean mine using white spirit vinegar. Put 3 measures of vinegar into reservoir neat. Run on steam as normal. Lots of crusty stuff appears in the food bowl bit. So much comes out that I usually do this twice (just reuse the same vinegar again, minus the bits obviously). Then run through twice with plain water, again 3 measures.
Works great, seems to make it run faster and hotter than when clogged up.
A word of warning though - nothing makes your house smell more like a chippy than boiling vinegar! Do it next to an open window!