Make sure you enunciate eg. remember that often has a t in it so say it, if it ends in -ing make sure you say it like that, don't forget the g or in some cases and a k
Ain't is not a proper word. I don't care who argues, you are supposed to say isn't. (As you may have guessed I dislike how people destroy the english language simply by speaking)
Little things like that make you sound a lot better.
Don't use "weren't" and "wasn't" interchangably. 'Anything' and 'nothing', not 'owt' and 'nowt'. There is a 'd' in the middle of 'didn't, it's not 'di't', and wa'eh' is actually said 'water'. These are all the ones I inflict on people, and probably the reason two people this week have said "Oh, you're from the North!" after only one sentence.
Then again you're from London, so I don't know how many of those you actually do anyway. I think accent's a big part of it though; I'd never think of Jarvis Cocker, for example, as posh even though I can't remember ever hearing him use bad English.
I really thought there was a bug on my screen then!!
And yeah I get alot of stick because I'm from a rough part of Essex and I do sound very common. People assume you're thick which is really annoying, I wouldn't change the way I talk though. Sod everyone else