1. Cropping is your friend. Most pictures need about 7/8 cut out of it for it to look right with what your trying to go for. You will be amazed how zooming in on the part of the picture you want, the off-centering it a bit will add to your design as opposed to just showing the entire pic.
2. Transparency options always give a variety of surprise effects. When you want a change just scroll through the different transparency options and you just may be sold on one of the outcomes that you never even thought of.
3. Resolution is what most people screw up on. If your designing for a computer monitor (Powerpoint, web design) than 72 DPI will work, but if you are designing for print, try and design it around 300DPI, print quality is a lot better than screen quality. Blowing things up disproportionate always looks bad.
4. Layers are your friends. Do everything in layers. The more layers usually the more control you will have. Have 3 duplicate layers and make two invisible and then toggle them on and off to see what one you like best. Change the blending options on some layers to add borders or shadows or transparency.
5. Actions are your friends. You can record your steps of what you do to an image and then implement those actions on other images that are yours. Or you can download actions sets online. Just type in “Photoshop Actions” in Google and you’ll find all sorts of cool actions.