I used washing up liquid for a little while but had shed loads of trouble getting tires off, and breaking beads. It was because the rim was being corroded by all the salt in washing up liquid And the rough surface was gripping the tyre.
I switched to rema tiptop tyre paste (�11 quid for an actual bucket full.) Never had an issue since and if has nice corrosion inhibitors in it also. So my wheels look better too. I threw some in a tobacco tin and carry it out for trail puntures as well.
Helps when fitting mitas C02 on the back. My god theyre stiff tyres haha.
Look at this on eBay
http://www.ebay.co.uk/itm/182196729446