(embarassing?) photoshop pointers needed!

howdy!
sorry if this is going to be worded from a novice and/or ignorant standpoint, but i was wondering if anyone could face me in the direction of the photoshop sun here

i'm rather fond of the aged & oddly developed quality to old photos, most recently seen in iphone's "hipstamatic" app if anyone is familiar

what would be your advice and techniques, without trying to offend anyone's trade secrets, to accomplishing those effects in photoshop?

i guess i can always find colors and light textures to "overlay"on top of pictures or illustrations, but i was wondering what little nuances anyone might know to really integrate the elements together better or more naturally. the difference between making an effect really look like a part of the piece than dropping vapid hack layers you can see from a mile away.

thanks for your expertise! just looking to get my so-so skills a graduation to the next grade