To start with, you should be clear about what your priorities are. Price is not the only issue, particularly if you are taking on a leasehold property. Possible negotiation objectives might include an initial rent-free period while you fit out the premises, a break clause allowing you to end the lease early, and minimising any security you are required to give (such as a deposit or personal guarantee).
It helps if you know as much as possible about what the seller or landlord’s position: for example, whether they are in a hurry to do a deal and whether they have already had other offers.
You should make sure that you know as much as possible about the local property market and what prices or rents are being achieved for similar properties. A local chartered surveyor can help with this. A survey of the premises will identify faults that you can use in the negotiations.
You may want to ask your professional advisers to negotiate on your behalf. They will be less emotionally involved and have the right expertise and experience.