I generally add about 15 - 20% to the restaurant bill if I've had good service - more if it's exceptional but like others have said here, pay the waiting service a decent wage and don't expect diners (or whoever) to make up for the lack of it.

Pizza / take-out delivery - I'll generally round the amount up to the nearest £5 or £10.  London Cabs - I'll give a couple of quid and round up.

Having said what I did about paying a living wage, I'm off on my travels to Egypt next week where tipping for absolutely everything is expected.  Even to the extent that going into a loo at a tourist site involves tipping the guy on the door to make sure you get given adequate loo paper!!!  Adequate = more than two sheets!  Sometimes you only have to make eye contact (invest in a pair of dark dark glasses) to be asked for baksheesh.

And having said all that, I love Egypt and the people there so I can't have found it too bad...