I seriously cannot recommend enough, the
You Suck at Photoshop (there is some profanity here, so if you're offended, stay away!) series on YouTube.
It's hilarious and the tricks that are taught are really useful. Watch the videos! ALL THE VIDEOS!
I think my best friend in Photoshop is the layer blending options and also the opacity. Changing the opacity back and forth can really make a difference in the seamlessness of whatever image you're trying to manipulate.