How much do I tip a licensed tour guide for a full day service? how much do I tip for a four hour service?
Opinions will vary regarding tipping of tour guides. For me, if the guide is independent and you're paying a somewhat hefty sum for the service then, no I wouldn't tip -they're getting enough of my money already. On the other hand if the guide is hired through a company that pays the guide then, yes, if I was very pleased with the service I would tip the guide. Also for me it wouldn't depend on how long I used the guide, more on whether I was very satisfied and/or if the guide went above and beyond to make the tour a good experience for me. I have had both good and bad guides and I have tipped anywhere from 0 to 20% depending on the experience. Use your best judgement.
We had an unpleasant experience with a snotty guide at Golconda in Hyderabad, India. I gave him just a few of their money and told him it was for his kids. He had asked for a whole bunch of their money and had gotten huffy about it. Nancy's reply is a good one. Don't be held hostage by thinking you automatically have to tip everyone who is doing their job.
That's a very interesting question as culture on tipping varies wildly from culture to culture.
Generally speaking in Catalonia for example, tipping in a restaurant is really voluntary and there's not an immediate expectation from the waiter to be tipped like it's my experience in the US (pardon if it feels I'm generalizing). Furthermore, tipping in the restaurant will also vary whether it's a top-end restaurant or a mom-and-pop cafe, whether it's lunch or dinner, whether it's a bargain already or an average price and most importantly whether you feel the waiter/restaurant deserve it or not, whether they just did their job or went the extra mile for you. No tipping and no one will complain (some might frown but won't dare to say anything).
As per tour guides, I'd differentiate between licensed tour guides and freelancers (ie. free walking tours, so common these days in many European capitals). For the first group, I share pretty much Nancy's views too. Their prices are quite often hefty so unless they've gone faaaaaar above and beyond I wouldn't feel inclined to tip them at all. A simple and polite thank you is enough. For freelancers (and free walking tour companies) that's a different matter because their benefits can be quite low (or zero, in the case of the free walking tours) and they make ends meet by the tips of the tourees. Therefore a tip, only if it has been a good service, it's probably a must.
But then here it comes the amount... I've noticed that many Americans tip on a percentage while we normally tip on a specific amount (or fork). Thus, at a restaurant or a taxi, a common practice is to round up the bill: say it's 43.35€ you'd round up to 45€, say it's 10.14€ then 11€, say it's 582€, you'd probably round up to 600€. There's always a sort of correlation between the total amount and the amount tipped, but it's totally decided by you, not by any norm or social practice. For a freelance guide (or free tour), 10-15€ it's an average but 5€ it's also common and 20€ also seen.
I guess the basic premise is that you don't need to be tipped when you are "doing your job". Far gone are the times when the minimum salary didn't exist and in some professions workers lived out of tips because the wages were so low. These days, there's a minimum salary established by the Government, and reviewed annually, which every worker receives at minimum -regardless of his job/industry.
Have to say too that sometimes any sort of tipping it's completely out of the question as what you should do is to press a complaint instead. The term "service" is not always a concept understood by some waiters/owners/workers. This is a complain some Americans make when visiting over in Europe because in the US service tends to be more "servicial", and over here sometimes it looks like anything but. But again, that'd be food for another discussion.
Sorry for this long dissertation but this is a fascinating issue that's always interesting to discuss especially with people from other cultural backgrounds as often the views are quite different.
Thank you everyone for your comments and advise on tipping tour guides and restaurants. It has helped me so much