This is a guide - I encourage you to experiment and tweak the effects to suit your own tastes. You'll learn more by doing that, than from any tutorial.
1, Less contrast (optional really - i think the final effect is better with less contrast but it's not necessary and can be turned off later)
2. Creating a vignette
3. Editing the colour with Curves - editing the channels individually for more vintage colours
4. Importing old paper texture - gives the image a warmer, more tactile feel
5. Creating Light Leaks - my favourite feature of lomographic photos, this technique can look very authentic with more tweaking and experimenting with colours and blend modes
I use Photoshop CS3 in the video, but it should be almost exactly the same since version 7 (with the exception of the Shadow/Highlight tool, but that's an optional step).
Created with VirtualDub, CamStudio, Audacity and Photoshop CS3