Like everyone said above. Few other things I do...
1) Taking your photos from afar with a long lens to get the whole cake in focus better. I use my other half's SLR which is fairly basic and if I take photographs close up with his fixed lens, it can't keep the whole cake in focus. Sometimes I like that effect but for a full cake shot, I find taking it from a long way away and zooming in is easier.
2) If you have a Mac, check out the iPhoto editing software as well. Photoshop is awesome but a photographer friend recently told me how he uses the iPhoto software more for general colour editing. I was reluctant at first but it is much easier I think :-)
3) Lightboxes can be really cheap! There are loads of 'how to make a lightbox' online as well but my lightbox only cost about £40. It's a home lightbox and folds away flat with little pockets on it for the lights to keep everything together. Again, took some time to get the camera lighting right with it but it now comes out for every cake :-)
4) If you aren't already, use a tripod!! Especially if you are using slow shutter speeds, I think it's the only way to keep it from blurring (even with the super stable hands we decorators usually have). I put the camera on a tripod with a 5 second capture delay so there is absolutely no movement when the picture is taken.
5) I've also found it useful to buy rolls of cheap wallpaper to complement your cakes and give them a nice background. In the past, I used to use a contrasting background and then delete it but it was fiddly and I'm not sure I liked the effect as much. I figured you would usually align a cake to the venue it was going to be displayed, so why not do that with photographs...