In our testing we have found a sturdy stand with an attractive A4/US Letter sized sign to be the optimal presentation to maximise tips.
Many stands will work but below is the best stand we have found so far:
Some things to consider when choosing a stand:
How easy can you mount/remove your phone?
Long term you may want to purchase a dedicated iPhone that you can stick a more permanent mount to.
Is the base heavy/sturdy enough to prevent it from easily being knocked over?
Is the iPhone secure in the mount? Can it be easily removed/slip out?