I found the plug idea to be unlike I had expected. I pictured an actual plug, lol. No, I had some discharge on and off that was a big 'gooey' (sorry, tmi). My last few days, I just felt a lot of pressure and had trouble walking because of it. I also began having lots of bathroom trips with number 2. My doctor said that this is normal as the body tries to clear that out right before labor. Those were my major signs. Then out of nowhere, I had contractions. I had no back pain or belly pain before that, just the pressure in my vagina. But the good news, once contractions start, you usually have plenty of time. Mine started at midnight and we didn't go to the hospital until the next day as I waited for them to get to five minutes apart. Every woman is different though. Here is a really helpful article:
http://www.medhelp.org/pregnancy-health/articles/All-About-Labor/2210
Good luck and you are so close. Very exciting!!