Once again, I'm going to agree with someone and that'll be clocksandroses in terms of pricing and doing bulk orders for a fixed price helps. You can also add a +1$ sort of deal if they want to add more characters in the mix because I know people would like that.
A thing you could also do is holiday discount prices or special offers on certain days. You could also advertise your works on other places like Tumblr because a lot of people who do commissions for their characters can be found there. Personally, I've always been fond of artists who offer to also give the full drawing + the icons too so the person can put up the big thing in their journal or profile, crediting you and giving you more exposure all the while. (Does that make sense? I hope it does.) You can offer that in your bulk orders to sweeten the deal and make people more likely to get a full set instead of just 7 or 8.
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A thing you could also do is holiday discount prices or special offers on certain days. You could also advertise your works on other places like Tumblr because a lot of people who do commissions for their characters can be found there. Personally, I've always been fond of artists who offer to also give the full drawing + the icons too so the person can put up the big thing in their journal or profile, crediting you and giving you more exposure all the while. (Does that make sense? I hope it does.) You can offer that in your bulk orders to sweeten the deal and make people more likely to get a full set instead of just 7 or 8.