Bridal and formal gowns are a different matter.
Anything the girl wears underneath that dress is going to affect its fit. Regular undergarments have an impact on the fit, and specialty shaper garments make a bigggg difference.
Men should wear a belt and have their shirt tucked in when being fitted for dress pants.
When fitting, have your client relax. They will not be standing at attention while wearing this garment, unless you are fitting someone for a military uniform! Talking small talk will usually put a client at ease. Just be sure they are not looking down at you in order to continue the conversation. Obviously this will affect the hem.
Their arms should rest comfortably at their sides and they should look forward, casually. If you are marking the hem on a formal and you need the client to rotate a little, wait until they get situated before you resume pinning. Use lots of pins. They will be your roadmap when the garment goes into your sewing room.
One more thing, ALWAYS ask if the client is "doing ok". Standing still for long periods does a number on the blood flow, especially if the client has their knees locked. Try to avoid having your customer pass out on the floor. It's just so messy!