Associate Work Experience

I think the proper question isn't "what work experience do top MBA's have that manage to land jobs in banking as associates?"  The proper question is "what work experience will get me into a top tier MBA program?"

Let's not put the cart before the horse, getting quality work experience will land you the top tier MBA, which will open up all sorts of doors as far as IB recruiting goes.  So to answer the second question, the answer is unfortunately "it depends."  Get the best job in a finance-related field that you can - commercial banking analyst program, internal finance at a Fortune 500, etc. and do well there.  In addition to your job, demonstrate leadership outside of the workplace to improve your chances of landing at a top tier school.  Once you're in the top tier B-school, they will provide the means and networking opportunities to land good IB positions (obviously, being well educated on subject matter and researching the industry is a given).  My buddy is at a second tier B-school (1 step below HBS/Wharton/SGB) that is still recruited by BB's.  He's the only one out of 65 students interested in banking that has prior banking experience.  My point is, to an extent it doesn't matter what you do, but that you do it well and round yourself out enough to land into a top B-school.

Also, to Bateman's point, I wouldn't necessarily give up on banking pre-MBA, if that's what you want.  If you can land something that's respectable - transaction services at a Big 4, corporate finance at a F500, for example, you may be able to lateral into some solid MM shops, even possibly BB.