Ideal ways of making a call for nominations
Improve the reward. If you're not currently offering money, consider doing it. Other forms of reward I can think of can be:


the opportunity to give a talk at a prestigious venue, to a nice audience
the opportunity to give a talk alongside a famous speaker (say, allowing the awardee to give a 15' talk just before a 1 hour talk by some famous person)
an introduction to some influent people, possibly looking to hire researchers