Ironically, we're happy to give discount pricing for a year deal - because in that case the customer is investing up front and we're confident once they're using the product they'll realize significant ROI and stay.
But re negotiations, I agree with Steli here - the customer who asks for a discount before they've ever tried the product will typically become a problem customer (especially for a young company trying to get the initial 10-100 customers).
Asking for a discount during your trial is one thing, asking for a discount before trying the product is another :)