I bought "Low Odour Kero", was about $18 for a 4 litre drum from Bunnings. I just cut a 20 Litre drum in half and soak larger parts in there and have some smaller cake tin size ones for smaller parts. Use a scouring pad to get the muck off.
Some good points of kero is: it doesn't evaporate, its not highly flamable, after use you can strain it and re-use it.
For larger aluminum parts which are stained and corroded (like say my inlet manifold), i couldn't really do much, took it round a few engine builders and none of them have a bath suitable for aluminium, tried another place, which from their ad in the yellow pages looked really good, however they have always been closed whenever I called or visited.
I resorted to then getting it bead blasted and plated, which originally I didn't want to do, but after speeking with them and seeing their work it was a good move. Costing rounghly $200 for manifold/plenum and fuel rail. This won't stain/corrode etc and it doesn't look half bad either. Will post pics when I get it back.
I had previously used brake cleaner and carby cleaner, but its not exactly practical for the price and I like my lungs the way they are.