Among other things suggested here, like adding more RAM, you could try using less resource-intensive desktop environment, like lxde or xfce, change to another display manager (gdm or kdm), and lessen the number of start-up processes.
Also, remember to clean up cache in firefox and occasionally do sudo apt-get autoclean and sudo apt-get autoremove to remove automatically installed packages that Ubuntu no longer needs. More info in man page for apt-get. There's some interesting cleaner programs in Software Center, like BleachBit.
To change desktop environment (the appearance), refer to this post: What kinds of desktop environments and shells are available?
To change to another display manager: http://ubuntuforums.org/showthread.php?t=1860704
For changing number of start-up processes and some other extra tweaks: http://www.howtogeek.com/115797/6-ways-to-speed-up-ubuntu/
Also, this last point is somewhat opinion-based, because I can't find the article I've read before, but it said that 64 - bit OS are generally performing much better. The test was done on 1 GB old Macbook, and 64-bit OS has showed better performance. So, when you want to upgrade to new Ubuntu release, consider switching to 64 bit version. It's not that you have to use 32 bit OS with less than 4 GB or RAM, but rather that 32-bit OS can't use anything above 4 GB, and 64-bit OS just differ in that - they can use more memory.
Occasionally, try looking into logs, see if there's any errors or crashes to troubleshoot problems. Generally you can look through /var/log directory, or through system log utility ( I personally have Ubuntu's default system log utility and installed KSystemLog , which i think is from for kde desktop ). Also, ocasionally, look which processes use up too much cpu/memory. For that use htop, top, or system monitor utility.
Good luck !
