buy a prescott cpu and leave on for 3 mins, should heat up everything inside perfectly
on a serious note, if I dont have a heatgun handy its either a soldering iron body [not the tip the bit immediately behind it] or a lighter
as far as mutiple goes allowing for cooling down, then it strikes me you've used the wrong size heatshrink, it should be one blast of heat on opposite sides of the heatshrink [along the length as well if its a long piece] and job done