Could you sell reserved parking spots to the offices? I'm guessing that the best spots for your patrons are different than the best spots for the office workers. If that's true, selling them reserved spots would make it seem more like they're buying something new rather than suddenly paying for something they're used to getting for free. You can probably talk to a sign maker in advance to see how much it'd cost to install reserved signs emblazoned with the company's logos.

When you approach them, I'd suggest having several options available and prepared in advance. Different sets of spots you're willing to sell them, options for how they'll be marked (plain signs, signs with their logo, lots painted with their logo). Make sure the most expensive option is overpriced to make the other options look more appealing, and to let them negotiate it down if they really want it so they can feel like they out-haggled you.