Viral marketing implementation


I released few days ago a website in software deals niche and I implemented a viral marketing system that helps vendors to promote software offers using their existing customer base.

Basically customers are announced that the software they use is on sale and from the deal page they can recommend the deal to friends using social network or email. They will be rewarded with a premium membership which gives them higher discounts and a longer promotional period for other deals if they bring 20 new registered users for ex.

I know that here are many experienced people in viral marketing, what are your suggestions regarding this matter?(1)
How can I make the most of it?(2)
What other things I should take into account?(3)
And a common questions, where is the best place where I could start finding users, what startup websites do you know?(4)


Thanks for your help

Maybe yes, I am in a dilemma on referrals number. Some users may give up and get disappointed if they tell about 30 friends and get almost nothing.

I think i will reduce it to 10 at the beginning, it will be much appealing also, however the effort of spreading the word will be lower.

Thanks for sharing your thoughts.

I think 20 new registered users is too high criteria.