1. Don't get lazy.
Keep looking even while negotiating for that perfect space you found. With a credible alternative, you can drive a harder bargain.
2. Find an architect before you look at space. He or she can tell you how much usable space you're actually getting and how much construction work it's going to take to get the office you want before you start negotiating.
3. Use a spreadsheet.
Use a spreadsheet to calculate the net present value of a deal, and so you can weigh one-time benefits against higher monthly rents. Often, landlords prefer to give you concessions up front -- a few months' rent free or money for construction -- if it means they can charge a higher monthly rent over the life of your lease. (Their banks like to see that they are charging high rents.) For you, this can be the same as taking out a loan, perhaps even at a negative interest rate.
4. Check the bathrooms.
I'm told it's amazing how many tenants sign leases without ever checking to see if the building has clean, well-maintained bathrooms.
5. Check the lot lines.
If you are in a city and your building is next to a vacant lot or a "taxpayer" (a one-story, low-rent building whose owner is content to hold on and pay property tax on the land until something bigger goes up on it), there's a good chance your views and natural light will go away when the lot next door finally gets developed.