All the objects on your desktop are loaded into ram so as to make them available to quicklook. It's best to have as few objects on the desktop as you can, stick them all in a folder is a good bet as they're only 1 level away then but not loaded into ram until you open the folder.
First, clean up your hard drive. It can be done manually but apps can make it easier. Download monolingual and delete all languages you don't want. Download the free versions of xslimmer and cleanmymac and run them after monolingual to get the rest. Download CCleaner for simple periodic cleaning (don't clean the cache too often as it speeds things up). Download onyx and learn it overtime, no rush. I think iFreemem is cool too, you can get it free with the trial stuff. Go through your app folder and get rid of everything you thought you'd use but don't.
Go into system preferences > accounts > login items and untick everything that makes no sense to you.
Run repair permissions with disk utility. Also run verify disk. If you have problems repair them by booting from your os disk. If you're on 10.6 or below download applejack as it's easier. Some say it works on lion as well but no idea. If you can't do either restart your mac and hold down CMD-S to get into single user mode. Type fsck -fy at the prompt and let it do its thing. Type reboot when it's done.
For more housekeeping, go through your preference folder and get rid of everything you no longer use (HD > library > preferences)
Drag activity monitor to the dock and run it when things are going slow to see what the offensive item is.
Open terminal and type 'sudo periodic daily weekly monthly' (without quotes), just to make sure it's done.
Every now and then i run disk inventory x just to have a look at what my hard drive looks like.
As for ableton i don't really know, but keeping projects self contained would be a good bet, i.e. all samples and audio files used in the project stored in the same folder. This will help prevent your os searching all over the place in different sectors for the files it needs. Freeze tracks to ease off on cpu usage. Use virtual memory on your samplers if you're running out of ram.