Step1:
I made this tutorial on a box mapped crate. This here is the wood texture i started off with.

Step2:
Get a new layer and write whatever it is you have to write.

Step 3:
Go to select>color range. For this example i used the black lines as that is where the most wear would occur.
I also added to the selection by slecting the next darkest color.
(This part takes some playing with sometimes to get the look your going for).

Step4: Once you've got your selection mask your ready to cut out your letters. So go to your text layer a press ctrl-x.
Take the layer oppacity down to about 85% and keep it on normal.
And here is your lovely crate. With lovely eazy wear and tear.