I had been using rubber bands and doubling sometimes tripling them up to make it the correct circumference but then i realized they were becoming so fat that it was risking popping off the linkage from the end of the carb, so i sought a solution i came across these:
http://www.amainhobbies.com/product_...oducts_id/7029
there are a couple bad reviews of them but so far for me they've been great, just figure out which of the many assorted sizes you need that it comes with, and profit.
although i will admit, the special spring that guy posted above looks dialed...probably a much much better solution, i think i'll order a few of those!!