Besides claiming that you are a young, growing, and cash strapped business that the landlord should help out of the goodness of his heart, I always found a few methods helpful.

1. The first one was to discuss the other spaces you are looking at using the real financial terms with the broker and or the landlord. Once they realize you might go elsewhere, they usually will attempt to cut to the chase and put their best foot forward.
2. The next technique is to attempt to phase your growth into the space. For example if the space is 2,500 square feet, you begin with the justification that you need 1,500 square feet now, but will need the 2,500 square feet a year from now. If you are signing a 3-5 year lease, the landlord should be willing to let you pay as you go which will strongly help the economics of what you are trying to accomplish.
3. Negotiate the rent as far as the landlord will move and then move on to concessions like free rent or tenant improvements.