Something which doesn't begin to reheat once it's used it's 'cooling power'. I dislike clays and gels because once they aren't cooling any more they're actually forming a nice insulating layer which will heat legs back up, especially on a hot day.
The best IMO is what rhino said- tubigrips and ice and cold hosing too. When Reg gets back XC he gets washed down, his legs get hosed a bit, he gets walked round. Return to lorry and repeat until they feel cool to the touch. Then tubigrips and ice while he munches Costs pennies and is very effective