You always compromise your negotiating position if the other side knows you really want what they have.
A friend of mine who is a landlord leasing agent told me that he was showing office space to a Fortune 100 company once in Dallas, Plano or Frisco - I can't remember which. When the company man got off the elevator and walked into the space he exclaimed, "this is perfect. I love it." Negotiations stopped right there. My friend knew he didn't have to make any further concessions.
So make sure the landlord (leasing agent) knows you are checking the market and considering several options. Never give them the sense that their space is the only one that works for your company. That is when the landlord pushes the profit button. You will never get the best deal available without competition.
This is especially true when you are renewing your lease. Landlords assume you aren't shopping the market even if you have a tenant rep working for you. So go look at other buildings. Ask for proposals from nearby buildings. Now you have a competition.
Keep the competition going even when there is only one competitor.