If you guessed crumpled paper you are correct! hee! Sometimes I don't like the way a layout feels when the paper is "flat". Sometimes it is just to "perfect" for the look I am going for. So the easiest way I have of getting it a little bit more worn - is to crumple the paper. There is no right or wrong way of doing so - just do it! And if you have time wet the paper and you will get an awesome look (I am usually not that patient).
When it comes time to adhere the paper I usually just put the adhesive onto the base and then place the crumpled paper on exactly the way I like. Then make sure to only press down in the spots that are flat so that you can keep the crumpled look!
That's it! I hope you use that technique - it really is simple and adds great dimension! Have a wonderful Sunday!