washing up liquid and water. Start opposite the valve as said above. Every time the tire gets to the point it's tight you need to go back round the bit you've already got onto the rim and make sure that both beads are pushed into the central well. Once you've checked that you should get another few mm's onto the rim, then go round again. Keep on doing it until you're there. Make sure there's no air in your tube too, that's stopped me in the past.
That's what I've learned from having to get dual ply HR's onto my FR2350 rims. It seems impossible at first. I still have to use tyre levers for the last little bit though. If I get a puncture on the trail you're going to hear a lot of swearing!