ONLY ACCEPT someone who will do Adwords aimed at the right customers. Therefore you are guaranteed real visitors. 50% after aft4er 2 days, 50% after 10 days.
The majority of the freelancers will promise you thousands of visitors per day. Truth is, they will not admit that the traffic are bots, not targeted, and will get your website penalized by google, harming your SEO and Alexa rankings. Their traffic is worthless if they are sending more than 100 untargeted visitors to your website without using Adwords or legitimate ads.
I will advertise for you via Adwords. I do this from my adwords account and pay for this myself as it is easier for me to control the campaigns for different clients this way.