Package deals from travel agencies are very, very pricey. On my safari(s) during my two weeks in southern Africa last Fall, I decided to do it myself. And it is very doable, IMO. You can check out my South Africa, Swaziland and Lesotho travelogs on my site below, for a recounting of my experiences.
However, here's some highlights:
South African National Parks website. You can pick any park (for example, Kruger) and actually book your campsites or huts up to a year ahead of time). You can book night drives -- all kinds of stuff -- online. There's an extensive message boards, too, with all kinds of advice.
The Baz Bus:
This is a cool way to get around the country (if you choose South Africa).
Swaziland National Parks website:
I stayed at Mlilwane park. You can do lots of hiking where you get pretty close to warthog, zebra, all types of antelope, hippo -- really cool. There's a hostel right on the premises that was real fun to stay at, too.
I've also heard (but not been there), that if you wait till you get to Kenya, and book your safari in Nairobi, you can save tons of money. Be careful, though, it's not nicknamed Nai-robbery for nothing!
Feel free to ask any questions, too. I had a great time in my 2 weeks, there...even despite the attempted mugging...!