I just purchased a bunch of textures online (but you can download some free ones on Flickr as long as you don't use for commercial purposes and give credit to creator).
Basically, you open your photo. Then add the texture photo as a layer and then can adjust its opacity.
By creating a layer mask, you can then paint over the areas you don't want to be covered by the texture. (black if you've got a white layer mask, or white if you've got a black layer mask).
You can add several masks, play around with saturation, opacity, etc and see what happens. After you're done, flatten your image and voila'! It's a fun way to put a little "umpf" into your photographs and add some new feeling to it wether its vintage tones, old paper textures, bokeh, etc.
Here's a before and after image after I added some scratches and vinate tones. Which do you prefer?