I tip 10 -20% in restaurants. 10% if service OK - 20% if the service is good, although if service / meal was poor I wouldn't hesitate to leave nothing(very,very unusual).
Let barstaff keep the change when I buy a round. eg; £22 bill, £25 paid - £3 tip
Let taxi drivers keep the change on a taxi journey. eg; £8 bill, £10 paid - £2 tip.
Shave my own head so don't need to tip a hairdresser.
I also tip the mechanic who services my car if the bills reasonable.(£10)
Recently found out that a local Indian restaurant I have visited for years - never passes on any of the tips left at the till to staff! Unbelievable!
In Holland my friend and I tried to tip staff for what was the best meal I'd had in years. We were working over there and had spent a good few hours in the staffs company - drinking(getting drunk). We felt we had taken up a full table for the night and tried to compensate them with a good tip(it was a small Italian family restaurant). They would have none of it said they'd enjoyed our company / and there was a service charge in the bill etc. It took some negotiating as they were adamant they wanted nothing extra - talk about culture clashes.
An ex-girlfriend who worked in New York every summer while at Uni - and was familiar with the US tipping culture. Once tried to reward poor service - obnoxious / unattentive waitressing / and poor food with a poor tip. The staff would have none of it and changed the gratuity on the bill and forced her(and friend) to sign for it. Followed by a tirade of abuse about British people!!!!!!!!!

Only in America, eh!