Marley - hi! We have been debating and debating this question! Here is one great thread with some basic explanations from Angie: http://youlookfab.com/welookfa.....in-slouchy
And then here is a follow up thread with some more visuals and my question about the backside, and other people's thoughts. http://youlookfab.com/welookfa.....in-slouchy
Speaking for myself, I have not been able to get a slouchy look to work merely by sizing up a few sizes. I do own the GAP boyfriend jeans and I did size up in those (as you did, right?) but those were ALREADY cut with looseness in the top and a very tapered bottom. If I try a regular skinny jean and size up a few sizes, so far (with the ones I have tried) on ME it simply looks like an all-over too-big jean. Not flattering at all.
But it seems to work brilliantly for others. Here is Angie in hers: http://youlookfab.com/2012/09/.....low-style/
Shannon followed up by ordering the same jeans and they also worked great on her different body type. So I'm not sure what is going on with me. Maybe I just haven't tried the *right* skinnies to size up in.
My absolute favourite slouchy skinnies are LESS baggy overall than my Gaps, yet still slouchy on me. They are the Banana Republic Heriitage slouchy skinny from several seasons back. http://www.shefinds.com/2010/b.....nny-jeans/
I sized up on those ONE size and so they hang lower on me (and I pretty much do need a belt to keep them on after I've been wearing an hour or so.) So there is some crotch drop. But not so much that it looks leg-shortening to any great extent.
In terms of what to wear with slouchy skinny jeans - well - anything! I like tucking and belting with mine, but a half tuck or pseudo tuck is also great. I even wear regular sweaters with my BR ones (with my Gaps I don't because I really only wear those in the summer....mine are quite lightweight).
ETA: I almost forgot: the FABRIC composition is going to be key. I tried on a pair of legging jeans yesterday, sizing way up, in the hope that they might work as slouchy skinnies. (Actually, I was hoping they would work any which way; I have another pair in my actual size and love them; these were on sale and were silver, so I was hopeful I could wear them one way or another.) Alas, no go. The elasticity of the fabric meant they still clung...sort of...but with huge wrinkles like elephant skin where the fabric was supposed to be stretching out. They just looked DUMB, not slouchy. So I had to pass.
ETA 2: My other thought - with your slim, straight shape, it might be worthy trying on jeans intended for curvier figures, as Angie has done. These might give a slouchier look. I know that my Kut from the Kloth Diana skinnies actually fit me much more like slouchy skinnies than regular skinnies. They are quite baggy in the bum (after about 10 minutes wear) and very loose in the thigh. These are in my regular size, and in fact, when I first modelled them, some on the forum worried they might be on the small side for me. But that is absolutely not the case. I quite like them at their current looseness.