[Joe]

It might also help with the AF system to try and look for contrast differences to lock the focus on to. To explain, most of the better digital compacts through to digital SLR's have a passive AF system. Passive systems rely on contrast detection, so it often speeds up auto focusing if you can lock onto, for example, the edge of an object where the background is lighter (or darker) so the sensors can basically optimise the contrast 'line' between that lighter and darker part of the picture. It's also one reason why passive AF systems can slow up quite significantly in low light (ie low contrast), or indeed with slower/darker lenses, particularly zoom telephotos, where focus is made even more critical....Now of course many of the new cameras have focus aids like sending out short fast bursts of light (which also doubles as 'anti red-eye functions) to help the AF sensors out. Many newer flash systems now have Infra-red (active) AF assist functions to help speed up auto focusing in low light (indoor parties being a prime example).
Sounds daft, but shining a torch at the subject also helps AF systems too!
Hope this makes sense, and might be a factor why the AF is sluggish sometimes?