Currently work as a server in a hotel restaurant & bar whilst studying, as mentioned previously we do get paid a decent enough wage to survive on with a few treats every now and again.
Tipping in our place is generally proportionate to the service, if I work with one of the others who puts a bit of effort in and makes our guests feel like they are being looked after we do well, if it's two numpties serving who don't have any social skills (the biggest problem is them not being able to tell when they are being annoying) we won't do so well. I'm not sure why they don't realise putting a bit of effort in directly benefits everyone.
This does make me more of a stringent tipper when I'm out though, in a restaurant or bar I find I'm more critical of the server and unless they impress me I don't really leave a tip.
Very occasionally tip takeaway delivery drivers (the rule I have with our local domino's is if they can bring 3 correct orders in a row I will give them a decent tip, they don't get it often!). In a taxi I usually just round up to the nearest pound to save messing with lots of coins.