The Selous is a great location, so you in for a great safari. A few tips that you may find useful.
Back Button Focusing
This is almost a right of passage for wildlife photographers. It separates the focusing from the shutter release button to the * or AF-ON button. This allows you to capture a range of situations without changing any settings.
Once set, put your focusing mode to AI servo, as you can use the following techniques to capture images.
If you want to focus and recompose, you simply focus with the */AF-ON button before taking you finger from the button. Then recompose before releasing the shutter (half depressing the shutter no longer causes it to refocus)
It would also allow you manually focus too.
If you subject quickly moves off, then push the AF-ON/* button and you can start tracking your subject. Then push the shutter when you want to capture the action.
Will look up the 550D manual online to give the correct change settings.
Depth of Field
Can be a real killer in the field, especially when you get caught up in the moment in a dynamic environment. It is important to always be considering the three way relationship between the following
1. Distance from your subject
2. Your Aperture
3. Focal Length
For example, those close ups at 400mm are going to require you to stop down, else the lions eyes will be in focus, but the nose not.
The Magic Number 7 -
The brain is able to remember between 4-7 decisions in a process. Treat each picture like a Golf Routine, and it become an ingrained part of your approach. A good starting point would be something along these lines;
1. Select Aperture
2. Check Shutter Speed (in the viewfinder before you take the shot)
3. Adjust ISO - To get shutter speed or for desired effect
4. Check Light Direction
5. Exposure Compensation
6. Apply Rule of Thirds
The Rule of Threes
I teach people to use this idea when capturing an images. Look for to include a minimum of three elements which can also be implied.
I also include overarching themes in this category.
Plan Your images
I would recommend thinking of a list of images you want to capture. Go one to 500px or flickr and try to understand different images you may want to capture. For example, elephant with trunk in the air.
Once you have made a list, jot down things like exposure compensation, shutter speeds, aperture, little techniques.
In my experience, if you come across a similar scene, it will enable you react, already knowing what kind of settings you want to be using.
100-400
The sweet spot of this lens is at F8, particularly @ 400mm. I would recommend trying to shoot at this aperture, though only do so if you are not pushing you ISO to high and introducing a lot of noise.
On a 550D - probably ISO800 and above?
Camera Stability
Which camp are you staying at?
In open vehicles, as you get in Selous, bean bags are far less useful if you are not sitting next to the driver.
I attach a manfrotto superclamp to the roll bars of open vehicles, then attach a tripod head.
Ebook
I have a free ebook on photographing wild dogs, of which Selous has a great reputation. It also covers a lot of the technique covered above, which you could apply to all wildlife.
http://rjohnsonimages.com/blog/?p=638
Finally
Just enjoy it....! Africa is a wonderful destination, I am more guilty than most of getting stuck behind the lens.