Having done this hike myself since I first started this thread, I have some tips of my own. BlueDevil's post covers a lot of the key points, but I'd like to emphasize some of them and SOMEWHAT disagree with others:
1) Unless you're young and extremely fit (which is a description I don't think applies to most of us), my biggest piece of advice would be NOT to attempt this hike in just 2 days. You MIGHT make it but you will most likely wind up EXTREMELY sore. And you might NOT make it and then you'll somewhat feel in a way like you "wasted" your efforts and STILL wind up EXTREMELY sore (the one consolation being that your efforts would not really be completely wasted since it would still be a very scenic hike). Even if you're extremely fit, why rush it? So you can get back to SJ to bang pu*sy? Okay, that might seem like a very good reason. However, IMHO, if you're going to expend this sort of effort, you might as well take full advantage of it. Spend day 2 climbing from the Crestones base camp to the summit and back, doing local hikes and relaxing and recuperating. Then take a third day for making the descent from the base camp, which I can attest can be just as difficult as the hike up (the ascent is rough on the muscles and lungs, the descent is rough on the joints). Plus the descent will be more dangerous if you attempt it with sore muscles and wobbly legs as it is easy to stumble over rocks, twist ankles, and/or slip on the muddy slick spots (particularly towards the very end).
2) Pre-training will surely help but keep in mind it is not just a matter of muscles. Altitude will play a big part. I didn't have any problems as severe as VlueDevil's friend but I was huffing and puffing. If you have a car it is certainly possible to stay in San Isidro like BlueDevil and his friend did for his first attempt. However, that means having to get up even earlier the day of your hike. Also, as BD pointed out, San Gerardo is actually quite a pleasant (albeit small) mountain town PLUS it allows you to partially pre-acclimatize at a higher altitude than down in San Isidro. BTW, speaking of pre-conditioning, be sure your hiking boots are well broken in before you go.
3) Getting reservations in advance from the US can be somewhat tricky. However, I also did my hike over Thanksgiving, did NOT get my reservation in advance, and had absolutely NO problem getting my permit when I arrived in Canaan (the village just below San Gerardo where the ranger station is located) by the late afternoon (5PM) bus the evening before my hike. For others, IF you go during the PEAK season and are NOT able to make your reservation in advance (most likely), it is less likely that you'll just be able to show up the night before and get your permit. In THAT case, you should try to get to Canaan as early as possible the day before AND be prepared for the POSSIBILITY that you will not be able to get a permit for the next day. In that worst case scenario, you shouldn't have any problem getting a permit for the next day but that will mean having to spend an extra night in San Gerardo. OTOH, if you get to the ranger station in Canaan by 5AM the day before your hike (as BD suggested) in all likelihood you WILL get a permit, but that will mean an extra night either in San Gerardo or San Isidro, so it amounts to basically the same thing.
4) San Gerardo is a nice place to hang out. There isn't much to do there other than simply relax and enjoy the mountain air and scenery, but you could go on some easy local warm up hikes (unfortunately, the "hot springs" were closed when I was there, but don't expect any lavish facilities like the hot springs in Fortuna, this is just a local hot-spring fed watering hole).
5) Supplies are available in a small abasteria (sp?) in San Gerardo and the prices are not horrible, but the selection is very limited. I think you'd be better advised to purchase your supplies further ahead of time if possible in San Isidro or even back in San Jose.
6) You USED to be able to rent sleeping bags at the Crestones shelter but you can't any more (last time I'd checked the most of the few websites with any info on this hike had not been updated to reflect that). However, you can rent one in San Gerardo (if your hotel doesn't have them itself they can find one for you). If you're going to go with a rental bag, I recommend you purchase a sleeping bag liner (from your camping supply store back home). It doesn't add much weight, adds a little warmth and can come in handy on future trips (or stays at hostels, cheap hotels or other questionable places

).
7) You should bring plenty of sunscreen and raingear. Water along the hike is not really a problem, so don't weigh yourself down too much with that. There is a place to refill about halfway up (and down) and plenty of water at the Crestones shelter. However, one other thing I would add to that list of must-haves is WARM and DRY clothing. Even with raingear, you will probably arrive at Crestones WET and it can get extremely COLD up there, even during the late afternoon when the cold wet wind blows but particularly at night when you try to go to sleep. Crestones is a surprisingly nice shelter for such a remote and rustic location but it is NOT heated and has only limited solar electrical power. I had my longjohn underwear, t-shirt and flannel shirt on inside my bag and I was still curled into a fetal ball trying to keep warm over night. A wool cap and a pair of gloves would also be a very good idea. And don't forget DRY socks and extra plastic bags to keep all these things dry inside your pack.
8 ) If you plan to do this hike with a partner (whether one you travel with from the US or one you find in San Gerardo), be sure you're well matched up in terms of conditioning and ability, otherwise one of you will find the other to be a huge drag or else will be struggling to keep up. I did my hike solo and went at my own pace. I hiked small parts of it with other people I met at various places once on the trail, but went on ahead myself or, more often

, let THEM go on ahead whenever it called for it (I think I actually enjoyed my solo time the most anyway). If anything were to happen to you along the trail (unless you start really late and are particularly slow climbing up), there will nearly always be someone coming along before too long to assist or send for help. Oh yeah, the trail is clear and well-marked with very few trails splitting off, so having a guide is absolutely NOT necessary.