The following is just an inexperienced guess, so feel free to ignore it and seek other counsel, but . . .
I think the most important thing is to realize that negotiating pay takes a totally different mindset from programming. When programming you're trying to be creative and cooperative.
When negotiating pay you should have a competitive mindset. Your employer is to some extent your opponent and you're playing poker with him. The thing is -- that's OK (or at least it should be). If a potential employer is making it feel like you're pushing against him personally when you ask for higher pay he's being the jerk, not you (this is of course situational -- things might be different if you've already committed to work for X amount of time).
I don't agree about a competitive mindset for negotiating salary, actually. It should also be collaborative.
Don't say "Why don't you give me a pay raise?" but say "What can I do to earn £Xk more?"
If the answer is "nothing" - find another job.
If the answer is "just keep doing what you're doing and wait 10 years until you get promoted to the level where that's the salary" - find another career.
If the answer is reasonable, and achievable within a reasonable period of time, do it and get your pay raise.
Often though people will start providing more value without an agreement up front that it will earn a raise. In those cases a great boss might go ahead and give it to you spontaneously, but if that doesn't happen you need to know how to apply firm but friendly pressure.
Well, providing extra value without any kind of agreement as to what compensation you might get for it is, IMHO, a rookie mistake - and one you shouldn't make twice.
If you have confidence in your abilities, address the topic up front!
I think the most important thing is to realize that negotiating pay takes a totally different mindset from programming. When programming you're trying to be creative and cooperative.
When negotiating pay you should have a competitive mindset. Your employer is to some extent your opponent and you're playing poker with him. The thing is -- that's OK (or at least it should be). If a potential employer is making it feel like you're pushing against him personally when you ask for higher pay he's being the jerk, not you (this is of course situational -- things might be different if you've already committed to work for X amount of time).