1. If you have a lawn, then scarify it. This is like brushing you hair when you've been a bit slutty and it's got all matted (i.e. it's a bit painful but thoroughly satisfying). When you've done that, you can aerate it, either by inviting all your friends round for a party in their stilettos (fun) or a garden fork (not such fun).
2. If you have new lavender (see below) and want it to be bodacious then cut it back really quite hard by the end of this month. This way it will sprout from near the base rather than getting all woody and ugly.
3. Tidy up you lazy thing! - it's september and that means out with anything that looks tatty, and in with your bulbs (well, all your bulbs except for tulips, which should wait till November for planting)