Couple things off the top of my head...
There's a line to walk between "taking it easy your first couple years" and "burning yourself out." And it's something to take seriously. Everyone who went through a graduate program had those folks in their cohort that were smarter than they were and worked just as hard, but managed their time poorly. They started taking papers to conferences before they had the skills to do so, they loaded up on technical courses without the adequate prerequisites and they faked interests in subfields or topics they felt would be trendy for the job market. Most of those people don't finish and end up working in the private sector (for a good chunk of cash...but still...)
You want to be aggressive in graduate school, but you also need to be smart about it. There's a lot to learn. Always keep an eye on the job market, but keep the other solidly on your development within your program. Your letters are going to either open or keep doors shut for you, and those letters are largely determined by your performance/attitude within your program. Once that die is cast, I think the best generic job market advice is to plan on practicing that job talk non-stop for about 6 months.
Having only been on the market in one, I can't really contrast the american to comparative subfields. They're both going to be competitive. My intuition is that there's always going to be a certain level of demand across american politics subfields, whereas it may fluctuate across regions in comparative/ir (i.e. mideast is hot at the moment).