I use to giveaway all the SBD earned from my posts and those posts use to get good amount of Upvotes. There were posts that earned about 3-5 SBD and the I got about 110 Upvotes. That was the highest number of upvotes I guess.
Most people do not comment, they just upvote and hope they'd win.
Variable Rewards
If you would follow B. F. Skinner you could keep a database of the people who vote for you. Starting off you would describe it as random but give 100 percent reward. Gradually you would reduce the payouts but put them on a variable schedule. There are a few people who give 100 percent payouts and I vote for them daily. Having a "Random" payout will lead to subject exhaustion.