I'm trying to write a thick British accent (say, the kinda accent in Lock, Stock and Two Smoking Barrels, or Snatch). How do I go about doing it, other than using the obvious British slangs.
I'd suggest to be anywhere near credible you watch and read British publication / film - and not any American / Rest of world imitation British. Though while we're on the subject the notion of a British accent is a strange one as while it may exist, it is so fragmented that personally I'd volunteer it didn't. The accents that your talking about in those films are typically I belive East End London accents, maybe or maybe not 'Cockney' as traditionally you would have to live in earshot of the Bowbells ringing to count as one. That accent is so far removed from one only 50 miles away to the South it's untrue, their's much softer and posher for lack of a better word - more like the Received Pronouncation one associates with English Gents (see cod American drama). My accent for example is Northern English, vowels shortened, 'barth' vs 'bath' for example. The accent only 50 miles from mine in Liverpool os again hugely different, typically quicker and with many quirks or their own. And as for the Geordies, Inner City Scots, West Country folk, and Valley Boyos well! My point is that for an outsider it would be difficult to produce credible sounding accents - so I recommend like I said at the beginning that should you wish to write something like Lock Stock, watch / read it a lot and try and research similar media - and or should you be intersted in Cockney there will be numerous sites to help you out - though the key will be to use it sparingly!
Read some James Kelman. His style is just about perfect for writing in an "accent" (in his case, a Glaswegian accent). Here are threads to a couple of his short stories http://www.randomhouse.com/boldtype/0699/kelman/sstory.html http://www.richmondreview.co.uk/library/kelman01.html
In my opinion, that's a very tough question. My only suggestion would be along with those above - read up on it. Sorry I can't be of more help.