By popular request :P
Something I've found useful to go along with practicing each accent beforehand, and an extension on the choice of words, is to prepare a phrase that features words that stress the particular quirks of that accent; it helps to kickstart it, so to speak. By leading with the phrase, it becomes easier to launch into the accent.
'She got peg shet allooverha' is unfortunately the only phrase that springs to mind by way of example, but it serves to illustrate the point; it stresses the mutated vowels, clipped endings and blurring-together of words. I don't know precisely what this accent would be, but it's fairly obviously rural.
You can cheat a bit with an upperclass British accent - the kind of BBC english spoken by Stephen Fry - just ennunciate each word very clearly, and slightly slowly, and it will fall into place.
It also sounds as though what you are trying to achieve is less about accurately imitating an accent and more about making NPC's sound distinctive. Something to consider along those lines is word order. If whatever passes for the common tongue in your campaign is not the first language of your NPC, expect the words to be a bit jumbled, and verbs, adjectives and nouns used a bit haphazardly. Everyone knows what Yoda sounds like, but his froggy voice is only half the equation - the other half is the distinctive way he reverses his sentences. This can also lead back to the first point; its pretty hard to say something like 'Is good to be having' without at least a passing stab at something approximately Russian.
Verbal tics and quirks are also fun. Lisps are obvious, but aren't limited to hunchbacked assistants - 'Yearth, marthter'. They can also portray someone childlike, or simpering, or effeminate, or who's just been punched in the mouth, depending on where you put the emphasis. The way sentences are ended can say a lot - it runs rampant in anime, where half the cast of some shows ends up with a 'dattebayo', 'un', or whatever stapled on the end of each sentence. But for a westernised example does 'yess, preciousss' ring any bells?