First: Get informed legal advice from an experienced commercial lease attorney
First, have the right professional support and do your homework. Discuss your goals with an experienced business attorney skilled in the negotiation of commercial leases. Your lawyer will be able to help you from the beginning so you are not trying to undo something that is already signed. He or she will have special knowledge to use in reviewing and negotiating important lease terms. Your legal counsel can lead the negotiation and consult with you as things progress.
If you choose to attempt to preliminarily explore commercial lease terms on your own,  be sure never to agree to any terms or sign a lease without an attorney’s final review and approval. Any acceptance of terms you might tentatively agree to should always be “subject to your attorney’s final review and approval.”  Many brokers will be anxious to have you sign a Letter of Intent and will tell you it is non-binding and even include language to that effect, but they may still try to argue its terms against you when you later attempt to change them.
Second: Do your homework
In addition to getting legal counsel, do not hesitate to take the time during lease negotiations to consult with other professionals who may be able to provide important information you should have before making any final decisions. Such professionals might include tax accountants, real estate brokers, architects and space planners. If part of the lease agreement will include any redesign or upgrading of the space at your expense, be sure you have gathered any necessary estimates, bids or assessments early on.