Ryan's suggestion may be alright for some folks who take the time to install a new battery connector like those used in mobile sound installations. But everyone I have seen, wasn't!
One of the problems amateurs face in connecting to existing batteries is where (or how) to connect to the battery. I have always used a second nut for the existing connector, and I've never had a problem. A couple of times, I've had to replace the bolt with a longer one, but that is a minor gripe.
Some batteries have both posts and side mounts to make them more "universal" and this obviously gives you another option.
Power distribution is much easier today when you use the aforementioned blocks. They are the best solution for the AVERAGE amateur. I go beyond these, and I end up spending a lot more money. If your installation is a "no-holds-bared" approach, I'll be happy to send you photos.
The one thing I am REALLY adamant about is taking your time, and do things right. One amateur I have been helping, has spend some three months installing a system I could do in a few hours. It isn't that mine (or his) will be any more (or less) functional, it is a case of knowing your limitations.
It is probably in the TMI category, but I use the seven Ps as a rule. If you want to know what that is, e-mail me.