First you must decide what your aims are.
If you are hoping to raise awareness of a company's dodginess, it is important to be media aware. The Press are unlikely to go with you any further than reception, so check out access to the roof (i.e.external fire escape) and make sure you have a suitable sized banner with a snappy slogan.
Walk into the reception area and ask to speak to someone concerning the company's Ethical Construction Criteria/ environmental policies etc. Stay calm and try to reassure the poor receptionist who will probably think you're out to rob/ wreck the place. If it looks like they're stalling, that's your cue to get disruptive.
Spread out and create havoc. Your mere presence should be enough to stop most of them working (incidents of this nature are not exactly every day occurrences) . Pull out phone connections, turn off computers,etc. Sit on their desks, use their phones and faxes to contact press/mum. Keep an eye out for any interesting documents (these are few and far between) and take photographs. A fairly strong magnet will screw up any computer disc near it.
Have leaflets to give out to staff, and remember that they will probably freak easily, so stay calm and try and keep dialog flowing. If you fancy settling in and making the most of the opportunity for stimulating debate, lock on to a radiator.
Bike D-locks last longer than chains, but are rather cumbersome (and expensive), you should consider using handcuffs or thumbcuffs instead.
As with all actions, there is a risk of arrest. Good old Breach of the Peace is the most likely, and other favourites include suspicion of criminal damage and suspicion of burglary.