Unless you are waiting for over a year, I probably wouldn't do the IUD. They can be expensive and that's a bit invasive if you are just waiting a few months. And I definitely wouldn't do depo as that can really screw things up, I'm not crazy about the pill either. We personally stopped using protection completely at 6 weeks postpartum because we knew we'd have a tough time conceiving with my PCOS and messed up cycles. Before the 6 weeks postpartum, we used condoms...most of the time. (My OB actually wouldn't let me go on the pill until all of my bloodwork for clotting disorders was done and the results back.)
Staying on prenatals is a good idea. They certainly won't hurt anything. And it's really better to be on them several months prior to conceiving.
I also think that staying on metformin is a good idea, too. Just be careful on it until you want to get pregnant as it can help regulate your cycle and sometimes your cycle may change after a pregnancy (although neither metformin nor pregnancy changed much for me.) I personally only took metformin while TTC and wouldn't be comfortable taking it in the first trimester of pregnancy, although it supposedly can help prevent miscarriage, it can also deplete important stuff like folic acid, some studies have found, so if you do take it, take it at a different time from your prenatal. The metformin helps some women lose weight (personally I gained weight on it.)
My MFM didn't want me taking LDA while TTC but my fertility doctor was fine with it, so I did some cycles and not others. The cycle I got pregnant I didn't take it until I got a + test.
I didn't do anything special dietwise, and in fact started my subsequent pregnancy about 15 pounds heavier than I started my previous pregnancy...so far...I've made it 12 weeks further and only 7 days until my scheduled c-section. I wouldn't stress about that too much. (In fact, when I asked my MFM at my pre-conception appointment how much weight she wanted me to lose before I got pregnant, she shrugged and said, "none, your weight is fine, it won't make a difference as far as pre-e"...it turns out she was right about that though obviously losing weight probably won't hurt. I think a low carb diet would be alright at least in terms of helping you out with PCOS/insulin resistance, but I wouldn't worry about it too much.
Me (29) DH (30)
#1-Olivia Caetlyn-9-28-09-9-28-09, 23+2 wks, emergency classic c-section, class I HELLP, IUGR
#2- Lucas Oliver (rainbow baby)- April 2011, 36+2 wks, HELLP and pre-e free! (lovenox and LDA pregnancy)
#3-Matthew, late October 2012...mostly normal, 37 wks, (lovenox and LDA again)
My blog: http://www.butterflies-and-rainbows.blogspot.com/