Now there's a really good question. It's something that I still struggle with for every image that I edit. I've mentioned before that I consider myself a forced creative, in that I mean that creativity is something that I really need to work at to achieve as opposed to being naturally creative. So when I look at editing my images I need some structure to keep me on track.
Before I shoot a scene I try to get an idea of what I want the finished output to look like (the operative word here is "try"). Will it make a good black and white? Does it have a lot of colour that I want to make vibrant? Would HDR be a good choice? This is typically my starting point so that when I upload into Lightroom, where my workflow starts, I already have a pretty good idea of what I want to do.
So here's a brief overview of my workflow and how I break things down for a little bit more structure. I tend to take threefold approach which consists of 1) global adjustments; 2) selective adjustments; and 3) finishing adjustments. Here's what that looks like:
White Balance: Is the image too warm or too cool? Or is there a colour cast? White balance is a good place to start. Auto WB works pretty well on most advanced DSLRs, but sometimes it take a lunch break.
Exposure: Is the image too light or too dark? Whether you set the exposure yourself, or let the camera do it, in some cases they can both be wrong. This is where I use the histogram and clipped highlight/shadow warnings. Try to make sure that you don't have any data leaving the left and right sides of the histogram.
Contrast: Done with either the Contrast slider or the Curves tool. I shoot in RAW format which means that there's no processing done by the camera so a little initial contrast boost can help.
Sharpening: Again working with RAW files means that there's no initial sharpening so I typically do a little bit in Lightroom. However, I always do final sharpening a little later on in my workflow.
Noise Reduction: My ISO is never the same for every image, and sometimes even my lowest ISO setting can show some noise. I always take a quick look and add some noise reduction if necessary. Lightroom has pretty good tool for this.
Selective Adjustments: (These are different for every image, but here's what I look for)
Distracting Elements: Is there anything that needs to be removed? Something that you didn't see in the viewfinder? This is where learning to use the variety of tools in Photoshop can help like the Clone Stamp; Healing Brush; and my personal favourite, Content Aware Fill.
Light or Colour Adjustments: Enhancing specific areas of the image with colour or light. This can help draw the viewers eye into the image directing it to where you want by lightening certain areas and darkening others. Lots of ways to do this with the layer adjustments, layer masks and the brush tool. However, my favourite plug-in for this is Nik Software's Viveza 2.
Contrast Adjustments: I find myself more often than not making selective contrast adjustments to certain areas of an image to bring out more detail. Similar to colour and light adjustments these are done with layer masks and the brush tool.
Selective Sharpening: Things that have a lot of edges can take a fair amount of sharpening; however, if you also have softer areas in the image, you definitely don't want to apply to same amount of sharpening. Portraits are a good example, you want smooth skin, but sharp eyes, so you would only apply sharpening to the eyes in this case.
Finishing Adjustments: (I'm typically back in Lightroom for these)
Vignette: I like vignettes, some people don't, but I do so I add them to pretty much every image. Sometimes it just a slight effect, but it's a nice finishing touch.
Exposure: This kind of bookends my workflow. I'm not sure when I started doing this, but usually after applying the post crop vignette I like to bump the exposure up just slightly to offset the edge darkening that creeps into the centre of the image. Exposure does this a little better than the Feather slider so it's a bit of a balancing act for me.
So there you go. A little peek into my workflow and how I approach my editing. It's taken me a while to get a clear focus on this and it's something that has developed over time. Your workflow may be similar or it could be totally different. All I know is that this works for me. In the end, only you can decide for yourself what is going to work best for you, but if you're still struggling I'll offer these 2 final tips:
1) Learn as much as you can about various Photoshop techniques. Learn the most widely used tools, and learn about using layers and layer masks; and
2) Seek out photographers who are creating the images that you really like. Try to learn how they process their images. With YouTube, blogs, Twitter and other social media options it's not as difficult as you may think find a tutorial or to connect and ask a question.
Sometimes we don't think about the edits that we could, or should be making, because we don't know how to do them, which can become a vicious little circle. So hang in there, learn lots and try new things.