disk usage analyzer not adjusted after emptying of trash

I suggest you install bleachbit, close as many apps as you can (even switch to metacity so compiz is not running, close email apps, browser, chat app etc etc)

then run it as both root and yourself (both available in the menu), the items which are reported as "this will take a long time" avoid as they are not very effective in space reduction and take AGES

The rest are fine (watch the firefox settings if you use (crappy) firefox). Its pretty self explanatory. Let it run and you will harvest a lot of space (it also clears out the trash).

I personally don't think trash shows up as used space as it is essentially deleted but still resides on the partition so I can understand the confusion (i'd be the same)

If space is an issue you can try a few things to reduce used space:

1. sudo apt-get clean
2. Remove old unused kernels to gain ~120Mb per kernel
3. Uninstall OpenOffice and install Abiword if you only use Writer (you can also install gnumeric if yuo need spreadsheets too). These are a LOT smaller than openoffice and will save space
4. run: dpkg -l | less and remove apps you NEVER USE (tomboy, gimp, evolution (if you use thunderbird), vino+vinagre if you don't use VNC...the list goes on)
5. dpkg -l | grep xserver-xorg will show the MASSES of drivers in an Ubuntu install, if you do not use the hardware then you can remove the driver to save yet more space (if you use an Nvidia gfx card you can remove nearly all the video drivers, just keep the nv and vesa driver. If the system is a proper install then the vmware video driver is useless to you)

A stock install of Ubuntu is FULL of rubbish that not all users will need or want and you can reduce these to gain space.

You can also use:

sudo apt-get install deborphan; sudo apt-get --purge remove `deborphan`

this will remove orphaned packages which are no longer required.