Before i had other publishers on my site i used a small hack to reward every new user with 10 credits. This worked very good and i had a sign up convertion >5% . Now coupons can do the same but there is the problem with commission for my publishers.
Solution.. give away a free bundle of your files as a reward for sign up. Use "New signup email to user" and provide a download link to the zip file which you just store somewhere on your server. And don't forget to write about on your homepage