Speaking as a top with a pretty thick cock, a lot of this depends on what the top does. Your body can take something quite large, but it has to be introduced gently and gradually as you get accustomed to it. If the top does the right things, the bottom should naturally adapt.
There are two parts to "taking it." The first is getting it in without causing injury or too much pain, and the second is fucking without tearing him up inside. If someone is having trouble, here are some of the things I try:
1) First, always plenty of good quality, long-lasting lube. Not the crap that goes sticky. Don't hesitate to apply more as often as necessary. If I'm wearing wearing a condom, I find I have a lot less sensation of the degree of lubrication, so I need to pay extra attention.
2) If he's really tight, sometimes I stimulate his hole with my fingers first. As it responds and relaxes, I work in one finger, then two, and then three fingers. I make sure to work in plenty of lube, too. If he can take 3 fingers at once, it is usually not too difficult to get my cock in. But often he's not ready for full depth of penetration. So just the head goes in, then I gradually work my cock back and forth until his body is ready, and lets me in. Some guys, however, don't like being stimulated with fingers-- they want to feel a cock.
3) Sometimes I will make my cock soft of half soft. (If necessary, think about kittens, or _________.) Then I'll try to stuff in the head, holding my hand around the shaft. This lets him squeeze my girth down a bit, and since the cock isn't really hard, there is less chance for injury. I'll gradually work in more of it, as I simultaneously start to grow hard inside him. I'll let him get used to it, then slowly build up intensity of fucking motions, which will get me fully hard as he grows accustomed to it.
4) If the bottom is into rough sex, sometimes a distraction will make him relax his hole, and let me in. I joke about this, but I've honestly reached around, and pinched a guys tits really hard, and as he gasps had my cock pop right in.
5) Once in, I don't start hardcore fucking right away. I'll gently get it in to full depth first (or as deep as it is going to go). Sometimes that takes just some gently motion, a little back and forth, or like a corkscrew. Other guys respond to sustained, but not too strong, pressure. Once I'm in all the way, and he's not pushing back on me inside, I'll start fucking with gradually increasing intensity, making sure I don't overdo it.
6) Many bottoms go through sort of cycles, where they relax and tighten up, then relax again. If the bottom is having trouble taking it, the top has to work with this, and slow down when the bottom needs it.
As far as what the bottom can do:
1) Be relaxed, and work together. This is about making both of you feel good!
2) "Bearing down" can make you open up and take the cock in, but be careful that you don't get more at once than you're prepared for.
3) Some bottoms swear by poppers, and I have to admit guys do seem to take me a lot easier if they have a snort. I personally can't stand the stuff, but I let guys do what they have too.
4) I've had some guys tell me they practice with sex toys (dildos, butt plugs).
5) The first time around, you may have to limit the intensity or duration.
6) If it's just too big, or too uncomfortable, say so.