I do happen to have an 8 GB machine, and almost never reboot (only reboot for OS updates).

I run half a menu-bar full of resident helper apps, like Dropbox (a big one), Fantastical, ScanSnap, Xmarks (another big one), Transmit (another big one), Evernote, and more. I also keep Apple Mail running, mapped to a half dozen Gmail IMAP accounts. I have "geek tool" updating my desktop with iCal appointments and various ps outputs.

I'm running a local MAMP stack and local Django stack. I run a Parallels Windows 7 VM for testing things in IE and testing from Windows in general.

Other running software is usually Safari, Terminal, Sublime Text 2, Codebox, Source Tree, Sequel Pro, Adium. I run and quit Office 2011 every time I need to edit a document. I run and quit Aperture and Photoshop.

Using this command line to see memory used by processes:

    ps -x -o rss,command | awk '{sum+=$1} END {printf "%9.3f %s\n", sum/1024, "MB" }'

Gives 3000.535 MB for all of the above, 3235.000 MB after adding Word and Excel, or 3794.277 MB after adding Photoshop CS5 and iTunes streaming radio.

Makes me wonder what you're doing to run out of memory.