Well, this is true about all modems and connections. The speed you connect at is directly dependent on the following factors:
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distance between your computer and your ISP via the phone company (shorter distances = higher speed)
quality of phone line - this include: if the phone lines are recent or old; if they are hanging on street poles or underground; number of telephone devices connected on the same line and their quality; cable used and quality of the jack connections
modems used by your ISP, and the protocols accepted (see v.90 versus 56kflex) - you should use, if possible, EXACTLY the same type of modem your ISP use, or something they recommend
note that you can NOT connect at 56k. Phone regulations allow maximum 53k/s at the best physical conditions, so it is very hard to achieve this in reality. "56"k connections work only if your ISP has a digital connection to the phone company; in that case, you will be able to download at 56k maximum, but uploads are still limited at 33.6k.
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I was able to connect at 14,400 at best. I had my phone line changed to a digital trunk of the phone company, and it suddenly went up to 24,000. Next, I bought the same modem my ISP uses, an US Robotics Courier 56k($250 a piece) and now I connect at 28,800. At the office, which is half a mile away from the ISP, I get 36k easy.
There is no certain rules to be followed in order to have a faster connection. Just keep in mind these general guidelines and you'll be fine. Happy connecting and good luck!