My MacBook Air keeps alerting that the startup disk is full and I've deleted applications, iTunes and MobileSync backups. What else??

I don't consider myself an advanced Mac user but I'm really trying to use my Macbook Air much more than I do... but the biggest hurdle for me is that it's been alerting me that the startup disk has been full for several months now no matter what I do. I'm on 10.9.1 OSX and can't even update to the newer version because I'm always out of space. I have 4GIGs of memory and a 60GB SSD of which 52GB are in use. Using OmniDisk Sweeper, I see I have 14.6 GB in Library of which 8.1 GB is Mail and 5GB is Application Support (and I have nearly 480 MB in Caches and 191 in Containers, 131MB in Messages). As for the rest, I've deleted my iTunes content multiple times but my iTunes library STILL says 1.7GBs in Music. I also have 672MB in Movies which I moved to an external drive and trashed so I'm not sure what still exists here. I only have 2 movie files and I deleted them both! I also have 16.9 MB in hob, whatever that is.

I have 32.8 MB in Logs, 19.8 in Safari, 7.9 MB in Calendars, 5.6MB in Google, 4.3MB in Preferences (!!), and 1.6MB in GameKit (not sure what this is), 1.1MB in KeyChains.

I'm just not sure what's going on. I have everything backed up via SuperDuper on a huge external drive and when I delete on my Mac, it doesn't seem to "take." Is is syncing that's the problem? Are my iTunes music files from my iPhone reinstalling back on the Mac as are all my photos and movies? I don't have that checked in iTunes so that shouldn't be happening either.

Again, I'm not an advanced user so maybe I'm not completing a final step to make sure it doesn't sync with my other devices (I also have an iPad mini). I like to get messages on my Macbook Air when I'm not near my phone but if it means I have 131MB in Messages that I can't pare down, I'd rather not. I see no way to reduce the Messages file so that it's not so large. I definitely want to get a handle on this before I even consider upgrading to a Macbook with more RAM and a bigger hard drive.

Final thoughts: Should I delete my mail accounts and just check mail on the web? Can I delete most of the files in Application Support or leave them alone? What's the difference between the MobileSync files and the MSYNC files?

Thanks in advance for help!