One difficulty of traveling in other countries is knowing when and how much to tip. In some places a tip is not expected; in others, it's an insult.
Here are guidelines offered by American Express:
In London, unless a service charge is on the check, tip 10% to 15% at hotels and bars. The same for taxi drivers, but less to porters and doormen.
In Paris, look for the words service compris on bills, which means the tip is included. If it is not there or the bill says service non compris (tip not added), add 8% to 12% of the bill.
Tip a few francs to cloakroom attendants, tour guides and doormen. Air and rail porters charge by the item. Tip taxi drivers 10% to 15%.